U.S. Department of the Interior

U.S. Geological Survey

Aquifer Susceptibility in Virginia, 1998-2000

Water-Resources Investigations Report 03-4278

Prepared in cooperation with:

Virginia Department of Health

Office of Drinking Water
Cover image: Collection of water sample for age dating ground water by chlorofluorocarbon technique.

Photograph by David L. Nelms, USGS.
U.S. Department of the Interior

U.S. Geological Survey

Aquifer Susceptibility in Virginia, 1998-2000

By David L. Nelms, George E. Harlow, Jr., L. Niel Plummer, and Eurybiades Busenberg 

Water-Resources Investigations Report 03-4278

Prepared in cooperation with:

Virginia Department of Health

Office of Drinking Water

Richmond, Virginia

2003
U.S. DEPARTMENT OF THE INTERIOR

GALE A. NORTON, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director

The use of trade or product names in this report is for identification purposes 

only and does not constitute endorsement by the U.S. Government.

For additional information write to: Copies of this report can be purchased from:

District Chief U.S. Geological Survey

U.S. Geological Survey Branch of Information Services

1730 East Parham Road Box 25286, Federal Center

Richmond, VA 23228 Denver, CO 80225-0286

e-mail: GS-W-VArmd DC

Information about water resources in Virginia is available on the World Wide Web at http://va.water.usgs.gov
Contents III

Abstract ....................................................................................................................................................................................................... 1

Introduction ................................................................................................................................................................................................ 2

Purpose and scope ........................................................................................................................................................................ 4

Hydrogeologic description of regional aquifer systems in Virginia ....................................................................................... 4

Appalachian Plateaus regional aquifer system ............................................................................................................. 5

Valley and Ridge regional aquifer systems ..................................................................................................................... 5

Blue Ridge regional aquifer system ................................................................................................................................. 8

Piedmont regional aquifer system .................................................................................................................................... 8

Coastal Plain regional aquifer systems ........................................................................................................................... 9

Sampling locations ......................................................................................................................................................................... 10

Methods ........................................................................................................................................................................................... 10

Acknowledgments ......................................................................................................................................................................... 12

Aquifer susceptibility ................................................................................................................................................................................ 12

Determination of susceptibility of regional aquifer systems ................................................................................................... 13

Appalachian Plateaus regional aquifer system ............................................................................................................. 13

Valley and Ridge Carbonate regional aquifer system ................................................................................................... 14

Valley and Ridge Siliciclastic regional aquifer system ................................................................................................. 14

Blue Ridge regional aquifer system ................................................................................................................................. 16

Piedmont regional aquifer system .................................................................................................................................... 16

Coastal Plain Shallow regional aquifer system .............................................................................................................. 16

Coastal Plain Middle regional aquifer system ................................................................................................................ 17

Coastal Plain Deep regional aquifer system ................................................................................................................... 17

Relation between aquifer susceptibility and apparent ground-water age .......................................................................... 17

Young waters ....................................................................................................................................................................... 18

Old waters or paleowaters ................................................................................................................................................ 24

Relation between aquifer susceptibility and selected chemical constituents .................................................................... 26

Nitrate ................................................................................................................................................................................... 26

Volatile organic compounds ............................................................................................................................................. 26

Statewide assessment of aquifer susceptibility ....................................................................................................................... 31

Summary and conclusions ....................................................................................................................................................................... 36

References cited ....................................................................................................................................................................................... 39

Figures

1. Map showing regional aquifer systems and wells and springs sampled in Virginia ....................................................... 3

2. Generalized hydrogeologic section representing conceptualization of the Virginia Coastal Plain Province ............. 6

3-18. Graphs showing:

3. Atmospheric mixing ratios of chlorofluorocarbon-11 (CFC-11), chlorofluorocarbon-12 (CFC-12), 

chlorofluorocarbon-113 (CFC-113), and sulfur hexafluoride (SF6) ............................................................................... 11

4. Exceedance of threshold chlorofluorocarbon and tritium concentrations and susceptibility .............................. 15

5. Mixing ratios of chlorofluorocarbon-113 (CFC-113) and chlorofluorocarbon-12 (CFC-12) (A)

and CFC-113 and chlorofluorocarbon CFC-11 (B) for water from well PD-22 ........................................................... 19

6. Distribution of apparent age of young water fraction in binary mixtures .................................................................. 20

7. Distribution of young water fraction in binary mixtures ............................................................................................... 20

8. Percentage of young water in relation to apparent age of young water fraction in 

binary mixtures .................................................................................................................................................................... 22

CONTENTS
IV Contents

9. Total thickness of the confining units in relation to apparent age of young water fraction in 

binary mixtures .................................................................................................................................................................... 22

10. Distribution of chlorofluorocarbon (CFC) contamination .............................................................................................. 23

11. Relation between Fontes-Garnier adjusted radiocarbon age (Fontes and Garnier, 1979) 

and distance from the Fall Line by Coastal Plain regional aquifer system ................................................................ 25

12. Relation between total thickness of confining units and Fontes-Garnier 

adjusted radiocarbon age .................................................................................................................................................. 25

13. Nitrate concentrations in relation to chlorofluorocarbon (CFC) mixing ratios ......................................................... 27

14. Nitrate concentrations in relation to apparent ages of young water fractions in binary 

mixtures with old (pre-chlorofluorocarbon) waters (A) and to Fontes-Garnier adjusted 

radiocarbon ages (B) .......................................................................................................................................................... 28

15. Distribution by regional aquifer system (in Virginia) of nitrate concentrations greater than 

the U.S. Geological Survey (USGS) detection level of 0.05 milligrams per liter ........................................................ 29

16. Chromatograms for selected wells and a spring in Virginia from gas chromatography ......................................... 30

17. Peak areas in relation to retention times for volatile organic compounds (VOCs) from 

gas chromatography with an electron capture detector (GC-ECD) analysis for samples 

that exceeded susceptibility threshold concentrations (A) and for samples that did 

not exceed susceptibility threshold concentrations (B) ............................................................................................... 32

18. Distribution of volatile organic compounds (VOCs) detected in water samples by 

major land-use category .................................................................................................................................................... 33

19. Map showing susceptibility to contamination from near-surface sources ....................................................................... 34

20-21. Graphs showing:

20. Depth to top of first water zone and well depth in relation to exceedance and nonexceedance 

of susceptibility threshold .................................................................................................................................................. 35

21. Distribution of the ratio of the percentage and apparent age of the young water fraction in 

binary mixtures .................................................................................................................................................................... 36

Tables

1. Distribution of wells and springs sampled as part of the Virginia Aquifer Susceptibility study by 

regional aquifer system and aquifer, 1998-2000 ...................................................................................................................... 7

2. Virginia Source Water Assessment Program (SWAP) susceptibility determinations 

for ground-water sources .......................................................................................................................................................... 13

3. Summary statistics for average chlorofluorocarbon and tritium concentrations ............................................................ 14

4. Summary statistics for adjusted Fontes-Garnier radiocarbon ages ................................................................................... 24

5. Apparent ages from chlorofluorocarbons, tritium/helium-3, and sulfur hexafluoride 

dating methods, and adjusted radiocarbon ages .................................................................................................................. 46

6. Percentage of young fraction in water samples .................................................................................................................... 53
Contents V

Acronyms and Abbreviations

A 14C activity measured for the water sample

And adjusted 14C activity in the observed water

A0 initial 14C activity

AP Appalachian Plateaus regional aquifer system

Ar argon

BP before present

BR Blue Ridge regional aquifer system

14C carbon-14

cc/L cubic centimeters per liter

CCl3F chlorofluorocarbon-11 (trichlorofluoromethane)

CCl2F2 chlorofluorocarbon-12 (dichlorodifluoromethane)

C2Cl3F3 chlorofluorocarbon-113 (trichlorotrifluoroethane)

CFC chlorofluorocarbon

CFCs chlorofluorocarbons

CFC-11 chlorofluorocarbon-11 (trichlorofluoromethane)

CFC-11/CFC-12 ratio of CFC-11 to CFC-12

CFC-12 chlorofluorocarbon-12 (dichlorodifluoromethane)

CFC-113 chlorofluorocarbon-113 (trichlorotrifluoroethane)

CFC-113/CFC-11 ratio of CFC-113 to CFC-11

CFC-113/CFC-12 ratio of CFC-113 to CFC-12

CO2 carbon dioxide

CP Coastal Plain

CP-D Coastal Plain Deep regional aquifer system

CP-M Coastal Plain Middle regional aquifer system

CP-S Coastal Plain Shallow regional aquifer system

δ13C isotopic ratio of carbon-13 to carbon-12

∆t radiocarbon age of the water

Freon 11 chlorofluorocarbon-11 (trichlorofluoromethane)

Freon-12 chlorofluorocarbon-12 (dichlorodifluoromethane)

Freon-113 chlorofluorocarbon-113 (trichlorotrifluoroethane)

ft foot

gal/min gallon per minute

GC-ECD gas chromatography with an electron capture detector

GIS geographic information system

3H tritium

3H/3He tritium/helium-3

Halon 1211 bromochlorodifluoromethane

3Hetrit tritiogenic 3He

IAEA International Atomic Energy Agency

Max maximum

MCL Maximum Contaminant Level

Med median
VI Contents 

Mgal/d million gallons per day

mg/L milligrams per liter

Min minimum

MTBE methyl tert-butyl ether

n number of wells and springs sampled

N2 nitrogen

nd not determined

Ne neon

NO2+NO3-N nitrite plus nitrate as nitrogen

NO3 nitrate

NP not possible

PCE tetrachloroethylene

PD Piedmont regional aquifer system

‰ per mil

pg/kg picogram per kilogram

pmc percent modern carbon

ppb parts per billion

SDWA Safe Drinking Water Act

SNP Shenandoah National Park

SWAP Source Water Assessment Program

SF6 sulfur hexafluoride

T1/2 half-life

TCA 1,1,1-trichloroethane

TCE trichloroethylene

TU tritium unit

USEPA U.S. Environmental Protection Agency

USGS U.S. Geological Survey

VAS Virginia Aquifer Susceptibility

VBCH Virginia Beach

VDH Virginia Department of Health

VOCs volatile organic compounds

VPDB Vienna Peedee belemnite

VPI Virginia Polytechnic and State University

VR-C Valley and Ridge Carbonate regional aquifer system

VR-S Valley and Ridge Siliciclastic regional aquifer system
 Contents VII

CONVERSION FACTORS, DATUM, AND ABBREVIATED WATER-QUALITY UNITS

Water temperature is reported in degree Celsius (°C), which can be converted to degree Fahrenheit (°F) by the following 

equation: °F = 1.8 (°C) + 32

Stable isotope ratios are reported as δ values computed from the formula

where Rx is the ratio of the isotopes measured in the sample and RSTD is the isotope ratio in the reference standard. The value of δx is in parts per thousand (per mil).

Vertical coordinate information is referenced to the National Geodetic Vertical Datum of 1929 (NGVD 29); horizontal coordinate information is referenced to the North American 

Datum of 1927 (NAD27).

Abbreviated water-quality units: Chemical concentration is reported in milligrams per liter (mg/L) or micrograms per liter (μg/L). Milligrams per liter is a unit expressing the 

concentration of chemical constituents in solution as weight (milligrams) of solute per unit volume (liter) of water. One thousand micrograms per liter is equivalent to one 

milligram per liter. For concentrations less than 7,000 mg/L, the numerical value is the same as for concentrations in parts per million. Specific electrical conductance of water 

is reported in microsiemens per centimeter at 25 degrees Celsius (μS/cm). 

Additional abbreviated units used in this report: L (liter), mL (milliliter), kg (kilogram), pg (picogram), fmol (femtomole), pptv (parts per trillion by volume), STP (standard 

temperature and pressure, 0 degrees Celsius and 1 atmosphere), cc/L (cubic centimeters per liter).

Multiply By To obtain

Length

inch (in.) 25.4 millimeter

inch (in.) 2.54 centimeter

foot (ft) 0.3048 meter

mile (mi) 1.609 kilometer

Area

square mile (mi2) 259.0 hectare

square mile (mi2) 2.590 square kilometer

Volume

gallon (gal) 3.785 liter

gallon (gal) 0.003785 cubic meter

Cubic inch 16.387 cubic centimeters

picocurie per liter 37.04 becquerel per liter

picocurie per liter 0.3135 tritium unit (TU)

ounce 28.35 gram

pound 0.4535 kilogram

picogram 1x10-12 gram

femtogram 1x10-15 gram

δx

Rx

RSTD

------------ 1 – ⎝ ⎠ ⎛ ⎞ = 1 000 ,
Abstract 1

ABSTRACT

The U.S. Geological Survey (USGS), in cooperation with the Virginia Department of Health, 

sampled water from 171 wells and springs across 

the Commonwealth of Virginia between 1998 and 

2000 as part of the Virginia Aquifer Susceptibility 

study. Most of the sites sampled are public water 

supplies that are part of the comprehensive Source 

Water Assessment Program for the Commonwealth. The fundamental premise of the study was 

that the identification of young waters (less than 

50 years) by multiple environmental tracers could 

be used as a guide for classifying aquifers in terms 

of susceptibility to contamination from near-surface sources. Environmental tracers, including 

chlorofluorocarbons (CFCs), sulfur hexafluoride 

(SF6), tritium (3H), and tritium/helium-3 (3H/3He), 

and carbon isotopes (14C and δ13C) were used to 

determine the age of water discharging from wells 

and springs. Concentrations of CFCs greater than 

5 picograms per kilogram and 3H concentrations 

greater than 0.6 tritium unit were used as thresholds to indicate that parts of the aquifer sampled 

have a component of young water and are, therefore, susceptible to near-surface contamination.

Concentrations of CFCs exceeded the susceptibility threshold in 22 percent of the wells and in 

one spring sampled in the Coastal Plain regional 

aquifer systems. About 74 percent of the samples 

from wells with the top of the first water zone less 

than 100 feet below land surface exceeded the 

threshold values, and water supplies developed in 

the upper 100 feet of the Coastal Plain are considered to be susceptible to contamination from nearsurface sources. The maximum depth to the top of 

the screened interval for wells that contained 

CFCs was less than 150 feet. Wells completed in 

the deep confined aquifers in the Coastal Plain 

generally contain water older than 1,000 years, as 

indicated by carbon-14 dating, and are not considered to be susceptible to contamination under natural conditions. All of the water samples from 

wells and springs in the fractured-rock terrains 

(the Appalachian Plateaus, Valley and Ridge, Blue 

Ridge, and Piedmont regional aquifer systems) 

contained concentrations of CFCs and 3H greater 

than one or both of the thresholds. Because all of 

the water samples exceeded at least one of the 

threshold values, young water is present throughout most of these regional aquifer systems; therefore, water supplies developed in these systems are 

susceptible to contamination from near-surface 

sources. No relation between well depth and presence of CFCs is evident from samples in the fractured-rock terrains.

More than 95 percent of the samples for which 

the dating methods were applicable contained 

waters with apparent ages less than 35 years. 

About 5 percent of these samples, most of which 

were from the Blue Ridge and Piedmont regional 

aquifer systems, contained young waters with 

apparent ages of less than 5 years. Most of the 

samples from the Valley and Ridge Carbonate, 

Blue Ridge, and Piedmont regional aquifer systems had young water fractions of more than 50 

percent, whereas samples from the Coastal Plain 

Shallow and Appalachian Plateaus regional aquifer systems contained less than 40 percent young 

waters.

Concentrations of CFCs in excess of air-water 

equilibrium, which can indicate that nonatmospheric sources (such as sewage effluent) have 

introduced CFCs into the ground-water system, 

were measured in 6 and 48 percent of the water 

samples from the Coastal Plain and fractured-rock 

regional aquifer systems, respectively. The nitrate 

(NO3) concentrations greater than the USGS 

detection level of 0.05 milligrams per liter generally increase as the apparent age of the young 

water fraction decreases, with the highest NO3

concentrations for samples in which one or more 

Aquifer Susceptibility in Virginia, 1998-2000

By David L. Nelms, George E. Harlow, Jr., L. Niel Plummer, and Eurybiades Busenberg
2 Aquifer Susceptibility in Virginia, 1998-2000

of the CFCs are above modern atmospheric 

mixing ratios (commonly referred to as “contaminated” for ground-water dating purposes). Most of 

the samples in which NO3 was detected were from 

the fractured-rock regional aquifer systems, especially the Valley and Ridge Carbonate regional 

aquifer system, where 90 percent of the samples 

had concentrations greater than the detection level. 

Numerous halogenated volatile organic compounds were detected at low concentrations (parts 

per quadrillion) in the samples from the regional 

aquifer systems in the fractured-rock terrains and 

the Coastal Plain Shallow regional aquifer system.

The ratio of the percentage and apparent age of 

the young fraction in binary mixtures of young and 

old (greater than 50 years) waters provides an indication of the relative degree of susceptibility. 

Large ratios are associated with samples in which 

the fraction of the young water is large (greater 

than 75 percent) and apparent age is extremely 

young (less than 5 years). Samples from the fractured-rock regional aquifer systems generally have 

the largest ratios. Analysis-of-variance tests indicate that the ratios in the samples from the Blue 

Ridge regional aquifer system are significantly 

higher (p<0.05) than those in the other regional 

aquifer systems. Results from the multiple tracer 

approach indicate that shallow wells (less than 100 

feet deep) and springs in the Coastal Plain and 

wells and springs in the fractured-rock terrains 

contain a component of young ground water and 

are, therefore, susceptible to contamination from 

near-surface sources.

INTRODUCTION

The U.S. Geological Survey (USGS), in cooperation with the Virginia Department of Health (VDH), 

conducted the Virginia Aquifer Susceptibility (VAS) 

study between 1998 and 2001 to determine the susceptibility to contamination from near-surface sources of 

the regional aquifer systems in Virginia that serve as 

public water supplies (Harlow and others, 1999). Water 

samples were collected from 165 wells and 6 springs in 

various hydrogeologic settings across Virginia (fig. 1). 

Multiple environmental tracers–chlorofluorocarbons 

(CFCs), sulfur hexafluoride (SF6), tritium (3H), and tritium/helium-3 (3H/3He)–and carbon isotopes (14C and 

δ13C) were used to determine the age of water discharging from wells and springs. Most of these wells 

and springs are regulated as public water supplies and 

are classified by the VDH as community, transient 

non-community, or non-transient non-community systems. Site and construction information for wells and 

springs sampled between 1998 and 2000 and the chemical and isotopic composition, recharge temperatures, 

and apparent ages of these water samples are presented 

in Nelms and Harlow (2003).

The Safe Drinking Water Act (SDWA) Amendments of 1996 require the Commonwealth of Virginia 

to develop and implement a comprehensive Source 

Water Assessment Program (SWAP). The VAS study is 

part of Virginia's SWAP (Virginia Department of 

Health, 1999), which is coordinated by the VDH, Division of Drinking Water. The fundamental premise of 

the VAS study was that the identification of young 

waters (less than 50 years) by multiple environmental 

tracers could be used as a guide for classifying aquifers 

in terms of susceptibility to contamination from 

near-surface sources. The information from the VAS 

study will be used by VDH (1) to determine which 

water supplies are sensitive to contamination from 

near-surface sources, and (2) to screen public 

ground-water supplies and identify those that require 

detailed source-water assessments.

Various methods have been used to determine the 

susceptibility and/or vulnerability of aquifers to 

near-surface contamination. DRASTIC, a 

ground-water vulnerability mapping method developed 

by Aller and others (1985), has been used throughout 

the world, including several counties in Virginia (Durnford and others, 1990; Hearne and others, 1992; Lynch 

and others, 1997; Melloul and Collin, 1998; Kim and 

Hamm, 1999). The DRASTIC method uses a subjective point-rating system that has been shown to have 

variable success when compared to actual 

ground-water-quality data (Barbash and Resek, 1996; 

Koterba and others, 1993; U.S. Environmental Protection Agency, 1993; Rupert, 1997, 2001). Ground-water 

vulnerability mapping in Great Britain has been done 

using a method that is similar to DRASTIC, but that 

includes detailed hydrogeologic variables (Daly and 

Warren, 1998; Palmer and Lewis, 1998).

Vowinkel and others (1994, 1996) and Storck and 

others (1997) developed numerical rating models to 
Introduction 3 Figure 1. Regional aquifer systems and wells and springs sampled in Virginia, 1998-2000.

39 ̊40 ̊38 ̊37 ̊

83 ̊ 82 ̊ 81 ̊ 80 ̊ 79 ̊ 78 ̊ 77 ̊ 

Fall

Line

Appalachian

Plateaus

Ridge

Blue

Piedmont

VIRGINIA REGIONAL AQUIFER SYSTEMS

Valley and Ridge

Coastal Plain Shallow, 

MIddle, and Deep

 Carbonate and Siliciclastic CP-03

CP-04

CP-02

CP-06 

CP-07

CP-01

CP-1

CP-19 C

CP-18

CP-22

CP-23

CP-24

CP-25 

CP-28

CP-29

CP-31

CP-30

CP-32

CP-33

CP-35

CP-34

CP-36

CP-39

CP-40

CP-38

CP-41

CP

CP-37

CP-44

CP-47 CP-46

CP-48

CP-50

PD-03

PD-04

PD-02

PD-05

PD-06

PD-07

PD-01

PD-08

PD-09

PD-11

PD-10 PD-12

PD-13

PD-15

PD-14

PD-16

PD-19

PD-20

PD-18

PD-21

PD-22

PD-23

PD-17

PD-24

PD-25

PD-27

PD-26

PD-28

PD-29

PD-30

VTDW-01VTDW-03AVTDW-03BVTDW-07AVTDW-07BVTDW-08

BR-03

BR-04

BR-02

BR-05

BR-06

BR-07

BR-01

BR-08

BR-09

BR-10

VR-04

VR-03

VR-02VR-06VR-07

VR-05

VR-01VR-11

VR-08

VR-09

VR-10

VR-12 VR-13

VR-15

VR-14

VR-16

VR-19

VR-20

VR-18

VR-21

VR-22

VR-23

VR-17

VR-24

VR-25

VR-27

VR-26

VR-28

VR-29

VR-31

VR-30

VR-32

VR-33

VR-35

VR-34

VR-36

VR-39

VR-40

VR-38

VR-41

VR-42

VR-43

VR-37

VR-44

VR-45 VR-47

VR-46

AP-03

AP-04

AP-02

AP-05

AP-06

AP-07

AP-01

AP-08

AP-09

AP-11

AP-10 AP-12

AP-13

NORTH CAROLINA

AP-01CP-50

VTDW-01

VB-01

COMMONWEALTH OF VIRGINIA

EXPLANATION

TENNESSEE

WEST

 VIRGINIA

KENTUCKY

WASHINGT

MARYLAND

VIRGINIA AQUIFER SUSCEPTIBILITY STUDY WELL AND NUMBERVIRGINIA AQUIFER SUSCEPTIBILITY STUDY SPRING AND NUMBERCITY OF VIRGINIA BEACH SHALLOW GROUND-WATER STUDY WELL AND NUMBERVIRGINIA POLYTECHNIC INSTITUTE AND STATE UNIVERSITYFRACTURED-ROCK HYDROLOGY RESEARCH STUDY WELLAND NUMBER

Roanoke

County boundaries from 1:100,000, TIGER files, 1994

Base from USGS Digital Line Graph, 1:2,000,000, 1987

Universal Transverse Mercator 18 projection, NAD 27, central meridian 75o00'W

Province boundaries modified from Virginia Division of Mineral Resources, 2003

0 25 

0 25 
4 Aquifer Susceptibility in Virginia, 1998-2000

determine the vulnerability of public drinking-water 

supplies to pesticide contamination in New Jersey. 

These models used geospatial, soil, and well-construction characteristics in conjunction with intensity of pesticide use, and were tested with a geographic 

information system (GIS) by grouping sites in terms of 

vulnerability (high, medium, or low) by using measured concentrations in water samples. Recent studies 

have classified aquifer vulnerability in terms of probability of occurrence of selected chemical constituents 

by the use of logistic regression methods (Tesoriero 

and Voss, 1997; Nolan, 1998, 2001; Rupert, 1998; 

Squillace, Moran, Lapham, and others, 1999; Frans, 

2000; Greene and others, 2002).

Detailed hydrogeologic information in Virginia is 

primarily limited to the aquifers in the Coastal Plain 

and Appalachian Plateaus Physiographic Provinces. 

The paucity of readily available hydrogeologic data 

makes it difficult to use numerical rating models and 

logistic regression methods to determine aquifer susceptibility on a statewide scale. However, previous 

investigations in Virginia used environmental tracers to 

identify young waters and, in some cases, indicated a 

correlation between apparent age and the occurrence of 

selected chemical constituents in ground water (Busenberg and Plummer, 1992; Dunkle and others, 1993; 

Nelms and Ahlin, 1993; Speiran, 1996; McFarland, 

1997; Nelms and Brockman, 1997; Focazio and others, 

1998; Plummer and others, 2001). The VAS study was 

designed to rely on measured concentrations of multiple environmental tracers in water samples collected 

from wells and springs that are representative of water 

supplies developed in the regional aquifer systems 

across Virginia (Harlow and others, 1999; Nelms and 

Harlow, 2000). The atmospheric origin of these environmental tracers increases their potential to be 

detected in ground water on a statewide scale. These 

tracers thus are more useful indicators of aquifer susceptibility than the occurrence and distribution of 

selected chemical constituents in ground water such as 

nutrients and synthetic organic compounds, which generally are related to near-surface point sources.

The term "apparent age," as defined by Plummer 

and Busenberg (2000), is used throughout this report 

because the model age from the environmental tracers 

approximates the time elapsed since a water sample 

was isolated from air in the unsaturated zone during 

recharge and is based on an analysis of measured concentrations of environmental tracers in ground water. 

Chemical (sorption and biodegradation) and physical 

(mixing and matrix diffusion) processes can alter the 

concentrations of the tracers in ground water and thus 

the model ages (Plummer and Busenberg, 2000; Shapiro, 2001). The initial apparent age represents estimates based on an individual environmental tracer 

whereas the final apparent age assigned to a water sample is based on an evaluation of the ages based on multiple environmental tracers.

Purpose and Scope

The purpose of this report is to present determinations of aquifer susceptibility to contamination from 

near-surface sources for the regional aquifer systems 

that serve as public water supplies across the Commonwealth of Virginia. The report presents the findings of 

the VAS study, which was conducted from 1998 to 

2001, and involved the use of multiple environmental 

tracers (CFCs, SF6, 3H, and 3H/3He) and carbon isotopes (14C and δ13C) to classify aquifers in terms of 

susceptibility to contamination. Water samples were 

collected from 165 wells and 6 springs to determine 

susceptibility of the regional aquifer systems. The 

report (1) classifies regional aquifer systems in terms of 

susceptibility, and (2) evaluates the validity of the VAS 

study methodology by comparing the occurrence of 

selected chemical constituents to aquifer susceptibility, 

apparent ground-water ages, and percentage of young 

water.

Hydrogeologic Description of Regional 

Aquifer Systems in Virginia

The Commonwealth was subdivided into eight 

regional aquifer systems on the basis of (1) physiographic province (Fenneman, 1938), (2) geologic province, and (3) hydrogeologic characteristics, (4) major 

rock type (in the Valley and Ridge Province), and (5) 

depth of the top of the first screened interval (in the 

Coastal Plain Province). These regional aquifer systems are:

1. Appalachian Plateaus

2. Valley and Ridge Carbonate

3. Valley and Ridge Siliciclastic

4. Blue Ridge
Introduction 5

5. Piedmont (including the Mesozoic Basins)

6. Coastal Plain Shallow (depths less than 200 ft 

below land surface)

7. Coastal Plain Middle (depths between 200 and 

400 ft below land surface)

8. Coastal Plain Deep (depths greater than 400 ft 

below land surface).

The Appalachian Plateaus, Valley and Ridge Carbonate and Siliciclastic, Blue Ridge, and Piedmont 

regional aquifer systems are characterized by fractured-rock aquifers. The carbonate rocks (limestone 

and dolostone) in the Valley and Ridge regional aquifer 

systems have different hydrogeologic characteristics 

(karst topography, solution channels, and caves) than 

the fractured siliciclastic rocks (sandstone, shale, and 

siltstone). The Coastal Plain regional aquifer systems 

are primarily composed of unconsolidated sedimentary 

deposits (porous media). Because of the wedge-like 

geometry of the Coastal Plain deposits, a single aquifer 

may be assigned to/classified in multiple regional aquifer systems (fig. 2) on the basis of hydrogeologic setting (Nelms and Harlow, 2000). The distribution of 

sites sampled during the VAS study by aquifer within 

the regional aquifer systems is summarized in table 1. 

The aquifer designations in the fractured-rock terrains 

are based on geologic formation whereas the aquifer 

designations in the Coastal Plain are based on aquifer 

nomenclature from Meng and Harsh (1988) and Hamilton and Larson (1988). The following sections contain 

descriptions of the regional aquifer systems and the 

corresponding physiographic provinces (modified from 

Nelms and others, 1997).

Appalachian Plateaus Regional Aquifer System

The Appalachian Plateaus regional aquifer system 

is within the Appalachian Plateaus Physiographic Province. This province encompasses approximately 1,600 

mi2 and is characterized by a well-dissected mountainous landscape with dendritic drainage formed on 

almost flat-lying to gently folded Paleozoic sedimentary rocks (Trapp and Horn, 1997). Elevations range 

from about 1,000 ft to more than 4,000 ft above sea 

level on High Knob in Wise County. The eastern edge 

of the province is the Cumberland Escarpment, which 

coincides with the Appalachian structural front where 

the westernmost fault of the Valley and Ridge Province 

reaches land surface. A thin mantle of weathered material (regolith) covers the bedrock, which is composed 

of relatively undeformed sedimentary rocks–sandstone, 

shale, and limestone (Powell and Larson, 1985). The 

most characteristic rock type is coal that occurs as beds 

and seams and is commonly fractured along joint systems (referred to as cleats) (Harlow and LeCain, 1991). 

Vertical stress-relief fractures are another prominent 

feature in this province (Wyrick and Borchers, 1981).

Ground water flows in (1) the regolith (unconsolidated alluvium, colluvium, and saprolite), and (2) 

along fractures and joints in the bedrock. Recharge to 

the fractured-rock aquifers is somewhat limited by the 

highly dissected, steep terrain and relatively impermeable bedrock. Ground water moves in a step-like manner along the more permeable vertical fractures and 

along cleats and fractures in the sub-horizontal coal 

and sandstone beds (Harlow and LeCain, 1991; Trapp 

and Horn, 1997). Downward movement of water along 

vertical fractures can be rapid and enhanced by pumpage.

Valley and Ridge Regional Aquifer Systems

The Valley and Ridge regional aquifer systems are 

within the Valley and Ridge Physiographic Province. 

This province encompasses approximately 10,600 mi2

along the western part of Virginia and consists of a belt 

of northeast-southwest trending ridges and valleys 

formed by the differential erosion of a thick sequence 

of folded and faulted Paleozoic sedimentary rocks (Pettijohn, 1970). Elevations range from about 380 ft above 

sea level where the Shenandoah River flows from Virginia into Maryland to 4,604 ft above sea level in 

southwestern Virginia (Butts, 1940). North of the city 

of Roanoke, Va., the province is part of the central 

Appalachians and is separated into two subdivisions: 

(1) a southeastern valley area, commonly referred to as 

the Great Valley, underlain by Cambrian to Ordovician 

carbonate rocks and shale characterized by broad valleys with interspersed ridges or hills; and (2) a northwestern ridge area underlain by Silurian to 

Pennsylvanian sandstones and shale characterized by 

high ridges with interspersed narrow valleys (Hack, 

1989). Another feature of the province north of 

Roanoke is a thick mantle of residuum, talus, and alluvial deposits that overlie the Cambrian carbonate rocks 

on the eastern slope of the valley at the foot of the Blue 

Ridge (King, 1950; Leonard, 1962; Hack, 1965, 1989).
6 Aquifer Susceptibility in Virginia, 1998-2000

Figure 2. Generalized hydrogeologic section representing conceptualization of the Virginia Coastal Plain Province as layered aquifers and confining units (modified from aquifer system classification used during the Virginia Aquifer Susceptibility study is also shown.

100

SEA

LEVEL

-100

-200

-300

VERTICAL SCALE GREATLY EXAGGERATED

METERS

WEST 

YORKTOWN-EASTOVER AQUIFER COLUMBIA AQUIFER

EXPLANATION

AQUIFER COASTAL PLAIN REGIONAL AQUIFER SYSTEM

Shallow (Depths less than 200 feet)Middle (Depths between 200 and 400 feet)Deep (Depths greater than 400 feet)

CONFINING UNIT

BEDROCK

SAPROLITE

DIRECTION OF GROUND-WATER FLOW

FRACTURES

COLUMBIA AQUIFER

YORKTOW

U

PIEDMONT

PHYSIOGRAPHIC 

PROVINCE

COASTAL PLAIN PHYSIOGRAPHIC PROVINCE

FALL LINE

-400

10

0

10 20 

0

AQUIA

SALTWATER INTERFACE

LOWER POTOMAC CONFINING UNIT

LOWER POTOMAC AQUIFER

AQUIFER

YORKTOWN-EASTOVER A

NANJEMOY-MARLBORO CO

NFINING UNIT

UPPER POTOMAC CONFINING UNIT

BRIGHTSEAT-UPPER POTOMAC AQUI

FER

CALVERT CONFINING UNIT

CHICKAHOMINY-PINEY POI

NT

AQUIFER

MIDDLE POTOMAC CONFINING UNIT

MIDDLE POTOMAC AQUIFER
Introduction 7

APPALACHIAN PLATEAUS VALLEY AND RIDGE

Carbonate Siliciclastic

Aquifer n Aquifer n Aquifer n

Bluefield 1 Conococheague 7 Brallier 4

Lee 1 Elbrook 3 Chattanooga 1

New River 1 Knox 1 Chemung 2

Norton 7 Martinsburg 1 Devonian/Silurian 2

Pennington Gap 1 Middle Ordovician 1 Granite 1

Wise 2 Reedsville/Eggleston/Moccasin 1 Huntersville 1

Total 13 Rome 3 Millboro 4

Shady 3 Millboro/Needmore 1

Waynesboro 1 Missippian Fms. Undiv 1

Total 21 Moccasin/Bays 1

Reedsville/Eggleston/Moccasin 1

Rome 4

Rose Hill 1

Waynesboro 1

Total 25

BLUE RIDGE PIEDMONT

Aquifer n Aquifer n

Augen gneiss 6 Amphibolite and Amphibolite rich foliats 2

Catoctin 1 Balls Bluff 3

Elbrook 1 Biotite gneiss 2

Granite 1 Everona 1

Granite Gneiss 4 Fork Mountain 1

Marshall 2 Garnet-biotite gneiss 1

Metamorphosed Sedimentary Rocks 1 Granite 1

Shady 1 Granite Gneiss 7

Total 17 Jurassic System 1

Manassas 1

Metamorphosed Sedimentary Rocks 2

Metamorphosed Volcanic and Sedimentary Rocks 3

Mylonite 1

Newark 1

Ordovician System 1

Petersburg 1

Vinita 1

Total 30

COASTAL PLAIN1

Shallow (depths less than 200 ft) Middle (depths between 200 and 400 ft) Deep (depths greater than 400 ft)

Aquifer n Aquifer n Aquifer n

Columbia 9 Chickahominy/Piney Point 7 Upper Potomac 2

Yorktown-Eastover 20 Aquia 1 Middle Potomac 8

Chickahominy/Piney Point 1 Upper Potomac 2 Lower Potomac 1

Virginia Beach 1 Middle Potomac 9 Total 11

Middle Potomac 2 Lower Potomac 1

Granite Gneiss 1 Total 20

Total 34

1 Depths are below land surface and aquifer system designation for a particular site sampled is based on the depth below land surface of the top of the first screened 

interval.

Table 1. Distribution of wells and springs sampled as part of the Virginia Aquifer Susceptibility study by regional aquifer system 

and aquifer, 1998-2000

[n, indicates number of wells and springs sampled]
8 Aquifer Susceptibility in Virginia, 1998-2000

This belt of unconsolidated deposits, commonly 

referred to as the “Western Toe” of the Blue Ridge, can 

exceed 600 ft in thickness (T.M. Gathright, II, Virginia 

Division of Mineral Resources, oral commun., 1994). 

South of Roanoke, the province is part of the southern 

Appalachians and is characterized by ridges and narrow valleys, and the predominant style of deformation 

within the province changes from folding to thrust 

faulting (Hack, 1989).

For this study, the Valley and Ridge Province was 

subdivided into two regional aquifer systems based on 

major rock type— carbonate (limestone and dolostone) 

and siliciclastic (sandstone, siltstone, and shale). The 

carbonate rocks in the Valley and Ridge have unique 

hydrogeologic features (karst topography, solution 

channels, and caves) that are not characteristic of siliciclastic rocks. Ground water in the carbonate rocks of 

the Valley and Ridge flows (1) in the regolith (alluvium, colluvium, and residuum); (2) along fractures, 

joints, and bedding plane partings in the bedrock; and 

(3) in solution channels and cavities (caves) formed by 

the dissolution of carbonate minerals. Ground-water 

storage in the carbonate rocks can be in the regolith and 

in the solution channels and cavities in the bedrock. 

Large volumes of ground water can move at relatively 

high flow velocities along the dissolution features in 

areas with low hydraulic gradients.

Ground water in the siliciclastic rocks of the Valley 

and Ridge Province flows (1) in the regolith (primarily 

alluvium and colluvium); and (2) along fractures, 

joints, and bedding-plane partings in the bedrock. 

Hobba (1972) estimated that circulation depths of 

ground water are over 1,500 ft in the thermal spring 

areas of western Virginia. Ground-water storage in the 

siliciclastic rocks can be in the regolith and in the fractures, joints, and bedding-plane partings. Flow velocities and gradients can be high and ground-water storage 

is limited in the fractured siliciclastic rock aquifers.

Blue Ridge Regional Aquifer System

The Blue Ridge regional aquifer system is within 

the Blue Ridge Physiographic Province. This province 

encompasses approximately 3,000 mi2 along a narrow 

northeast-trending belt between the Valley and Ridge 

and Piedmont Provinces and consists of a chain of 

mountains and highlands underlain by metamorphosed 

Proterozoic and Paleozoic rocks (Reed, 1970). Elevations range from about 220 ft above sea level along the 

Potomac River at the Maryland/Virginia state line to 

5,729 ft above sea level on Mount Rodgers. Generally, 

soils are thin and weathering profiles are shallow 

(Meyer and others, 1965). Hack (1982) separated the 

province into two sections because the origins of the 

topography differ. The section north of the Roanoke 

River, the Northern Blue Ridge Mountains, is characterized by a narrow range of high mountains underlain 

by Precambrian to Cambrian quartzite, phyllite, 

metabasalt, and granodiorite that form the northwest 

limb of an anticlinorium. The section south of the 

Roanoke River, the Southern Blue Ridge Province, is 

much broader and was separated by Hack (1982) into 

five subdivisions based on topography: (1) the Chilhowee-Walden Creek belt, underlain by Cambrian 

quartzite and faulted carbonate rocks and shale that 

form long, steep ridges separated by parallel valleys 

along the northwest margin; (2) the Mount Rodgers 

area, underlain by Precambrian volcanic and metasedimentary rocks that form a few high ridges just north of 

the North Carolina/Virginia border; (3) the Blue Ridge 

Highlands, underlain by massive Precambrian gneisses 

and amphibolites that form high mountains cut by deep 

valleys and basins; (4) the New River Plateau, which 

encompasses the headwaters of the New River in North 

Carolina, underlain by thinly layered schist and gneiss 

that form a broad plateau with a few low mountains; 

and (5) the Blue Ridge Escarpment, underlain by finely 

laminated gneiss that forms a narrow strip of land that 

drains southeastward to the Piedmont Province.

Ground water flows in (1) the regolith (unconsolidated alluvium, colluvium, and saprolite), and (2) 

along fractures and joints in the bedrock. Numerous 

springs discharge from the regolith and from the contacts between the regolith and bedrock. Flow velocities 

and gradients can be high and ground-water storage is 

limited in the fractured-rock aquifers in the Blue Ridge. 

Springs with source areas in the alluvium and colluvium on the mountain slopes can contain water with 

extremely short residence times, from 0 to 3 years 

(Plummer and others, 2001).

Piedmont Regional Aquifer System

The Piedmont regional aquifer system is within the 

Piedmont Physiographic Province. This province 

encompasses approximately 16,300 mi2 along the 

east-central part of the study area and consists of a gently rolling plain underlain by polydeformed and meta-
Introduction 9

morphosed Proterozoic and Paleozoic rocks (Fisher, 

1970). A thick mantle of soil and weathered rock 

(saprolite, regolith, or residuum) that overlies the fractured crystalline bedrock is a characteristic feature of 

the province (Meyer and others, 1965; Conley, 1985; 

Swain and others, 1991). Generally, regolith developed 

on the crystalline rocks is thick under the hilltops and 

thin to absent in the stream valleys (Richardson, 1982; 

Pavich and others, 1989). Elevations range from about 

200 ft above sea level along the eastern border to 1,000 

ft above sea level along the western border of the province. Hack (1982) noted that the Piedmont Physiographic Province of Virginia consists of the Foothill 

zone on the west, the Piedmont Lowlands on the east, 

and the Northeastern Highlands on the north. The Foothill zone also encompasses the southeastern part of the 

Blue Ridge Physiographic Province. Resistant volcanic 

and metamorphic rocks that form chains of isolated 

hills and ridges underlie the region of the Foothill zone 

north of the Roanoke River. The region of the Foothill 

zone south of the Roanoke River is underlain by resistant rocks of the Smith River allochthon that forms an 

upland (Hack, 1982). The Piedmont Lowlands are 

underlain by feldspathic gneiss and schist intruded by 

granitic plutons with lesser amounts of metasedimentary and metavolcanic rocks that form low ridges or 

hills and ravines.

Down-faulted Mesozoic sedimentary basins, which 

encompass approximately 1,340 mi2, are located within 

the Piedmont Physiographic Province. These basins are 

underlain by shale, siltstone, mudstone, sandstone, and 

basalt and are characterized by generally lower relief 

than the surrounding Piedmont (Hack, 1989, p. 461). 

Thin soils and shallow weathering profiles are characteristic features of the Mesozoic Basins (Froelich, 

1985; Conley, 1985).

Ground water in the crystalline terrains flows (1) in 

the regolith (predominantly saprolite), and (2) along 

fractures and joints in the bedrock. Most of the 

ground-water storage in the crystalline terrains is in the 

saprolite. Ground water in the Mesozoic Basins predominantly flows along fractures, joints, and bedding-plane partings. Because of the thin mantle of 

saprolite, ground-water storage in the rocks of the 

Mesozoic Basins is predominantly in the fractures, 

joints, and bedding-plane partings. Flow velocities and 

gradients can be high near pumped wells.

Coastal Plain Regional Aquifer Systems

The Coastal Plain regional aquifer systems are 

within the Coastal Plain Physiographic Province. This 

province encompasses approximately 8,500 mi2 in the 

eastern part of the study area and consists of an eastward-thickening wedge of sediments composed principally of unconsolidated gravels, sands, silts, and clays, 

with variable amounts of shells. These unconsolidated 

deposits directly overlie bedrock that slopes eastward 

and is commonly referred to as the “basement.” The 

sediments attain a maximum thickness of more than 

6,000 ft in the northeastern part of the Coastal Plain of 

Virginia. Onuschak (1972) states that the sediments are 

6,186 ft thick beneath the Eastern Shore Peninsula at 

Temperanceville, Va. Coastal Plain sediments thin 

westward and effectively end at the Fall Line, the 

boundary with the Piedmont Province. Elevations 

range from sea level in the coastal areas to about 250 ft 

above sea level near the Fall Line.

The regional aquifer systems in the Coastal Plain 

are subdivided into a series of aquifers separated by 

intervening confining units consisting of silts and clays 

(Meng and Harsh, 1988). Generally, the Columbia 

aquifer and, in certain areas, the Yorktown-Eastover 

aquifer, are unconfined and the deeper aquifers are confined, except in areas near the Fall Line where these 

aquifers are exposed at the surface (McFarland, 1997). 

In the lower Chesapeake Bay, the structural and stratigraphic features created by the Chesapeake Bay impact 

crater have disrupted the "normal" aquifer system 

sequence and influenced the hydrogeology, 

ground-water-flow system, and water quality in a large 

part of the Coastal Plain. The impact crater was formed 

when a large comet or meteorite crashed into shallow 

shelf-depth waters of the western Atlantic Ocean 

approximately 35 million years ago. The crater is a 

56-mile wide structure that penetrates the basement to 

a depth of at least 1.2 miles near the town of Cape 

Charles located in the southern part of the Eastern 

Shore of Virginia (Powars and Bruce, 1999; Powars, 

2000).

The Coastal Plain was subdivided into the following three regional aquifer systems on the basis of the 

depth of the aquifer and screened interval below land 

surface: (1) shallow (depths less than 200 ft below land 

surface), (2) middle (depths between 200 and 400 ft 

below land surface), and (3) deep (depths greater than 

400 ft below land surface). Because of the wedge-like 

geometry of the Coastal Plain deposits, a single aquifer 
10 Aquifer Susceptibility in Virginia, 1998-2000

may be assigned to/classified in multiple regional aquifer systems on the basis of hydrogeologic setting 

(Nelms and Harlow, 2000). For example, near the Fall 

Line, the Middle Potomac aquifer is classified in the 

Coastal Plain Shallow system, but the regional aquifer 

system designation changes to the Coastal Plain Middle system and then to the Coastal Plain Deep system 

as the aquifer becomes deeper towards the east and is 

less connected to hydrologic and anthropogenic activities at the surface.

Ground water flows around and between sediment 

grains in the Coastal Plain. This tortuous path causes 

the residence times to be much longer than in the other 

provinces. Ground-water storage can be much larger in 

the Coastal Plain than in the other provinces. About 

100 Mgal/d (million gallons per day) are removed from 

the Coastal Plain aquifers by major ground-water users 

(Hammond and Focazio, 1995). Flow in the shallow 

aquifer system generally is along local to subregional 

flow paths, whereas flow in the middle and deep aquifer systems is along subregional to regional flow paths.

Sampling Locations

Public water supplies, which are classified by the 

VDH as community, transient non-community, or 

non-transient non-community systems on the basis of 

water use and population, were the primary focus of the 

VAS study. Sampling locations were selected on the 

basis of (1) geographic position of the supply within 

Virginia and within the respective regional aquifer system, (2) availability of well-construction information, 

and (3) site accessibility (fig. 1). Some sites that are not 

classified as public water supplies were selected 

because of their unique characteristics. For example, 

the public water-supply data base maintained by the 

VDH did not contain any springs in the Coastal Plain 

of Virginia; therefore, spring CP-50 was selected for 

sampling to assess the susceptibility to contamination 

of springs in the Coastal Plain. The wells and springs 

that were sampled, which included wells for a shallow 

ground-water study for the city of Virginia Beach 

(Johnson, 1999) and wells for a Virginia Polytechnic 

Institute and State University fractured-rock hydrology 

study, were assigned VAS numbers (fig. 1). Site and 

construction information for wells and springs sampled 

between 1998 and 2000, and the chemical and isotopic 

composition, recharge temperatures, and apparent ages 

of these water samples are presented in Nelms and Harlow (2003).

Methods

The sampling and analytical methods used during 

the VAS study are described in detail in Nelms and 

Harlow (2003). Ground-water apparent ages were estimated by a multiple tracer approach whereby the environmental tracers–chlorofluorocarbons (CFCs), sulfur 

hexafluoride (SF6), tritium (3H), and tritium/helium-3 

(3H/3He)–are used to date young waters (less than 50 

years), and naturally occurring carbon isotopes–carbon-14 (14C) and carbon-13 (δ13C)–are used to date 

paleowaters (greater than 1,000 years). 

Ground-water dating with CFCs (Busenberg and 

Plummer, 1992) is based on Henry’s law of solubility, 

which is a function of the temperature during recharge 

and the salinity of the ground water. Concentrations of 

dissolved nitrogen (N2) and argon (Ar) were used to 

estimate recharge temperatures and excess air in 

ground water (Heaton, 1981; Heaton and Vogel, 1981; 

Heaton and others, 1983; Busenberg and others, 1993). 

In shallow ground waters, salinity corrections are not 

necessary because these waters generally contain dilute 

salt concentrations (Plummer and Busenberg, 2000). 

The apparent CFC age of the water sample was determined by comparing the calculated partial pressures of 

CFCs in solubility equilibrium with the sample to the 

historical atmospheric CFC concentrations in North 

American air (fig. 3). 

The SF6 dating method (Busenberg and Plummer, 

2000) is similar to the CFC dating method (fig. 3). 

However, unlike CFCs, SF6 in ground water can be 

derived from anthropogenic and terrigenic sources, it is 

generally not affected by sorption and biodegradation 

processes, and it can be affected by excess air in ground 

water, which is attributed to air entrainment during 

recharge (Heaton and Vogel, 1981).

Tritium (3H) is the radioactive isotope of hydrogen 

with a half-life of 12.43 years (International Atomic 

Energy Agency, 1981) and is an indicator of ground 

water recharged since 1952 (Clark and Fritz, 1997). 

Production of 3H in the atmosphere naturally occurs by 

cosmic ray spallation, but the principal source since 

about 1952 has been the atmospheric testing of thermonuclear weapons (fig. 3). Because 3H levels in water 

are not unique to a specific year, the 3H method was 
Introduction 11

used primarily to confirm the presence of young water. 

The 3H/3He method is based on the radioactive decay 

of 3H to 3He such that the helium isotope mass balance 

is used to determine the amount of tritiogenic 3He 

(3Hetrit) derived from 3H (Schlosser and others, 1988, 

1989). If the 3Hetrit is confined in the aquifer, apparent 

3H/3He ages of the water samples (τ) can be calculated 

from the following formula (Schlosser and others, 

1988, 1989):

where T1/2 is the half-life of tritium. Neon (Ne) concentrations are used to correct 3Hetrit for samples that contain terrigenic helium from crustal and mantle sources 

such as crystalline rocks.

Dating of paleowaters recharged over the past 

30,000 years is based on the radioactive decay of 14C 

(Clark and Fritz, 1997). Radiocarbon dating is problematic for two reasons: (1) the initial 14C activity (A0) 

at the time the water was isolated (recharged) from the 

ambient 14C reservoir is not known, and (2) the 14C 

activity measured for the water sample (A) does not 

indicate the effects of chemical reactions that might 

have altered the initial 14C activity along a flow path 

(Wigley and others, 1978, 1979; Plummer and others, 

1994). Adjustment models are applied to individual 

water samples to account for inorganic reactions that 

could have affected the inorganic carbon reservoir in 

the aquifers sampled (Plummer and others, 1994). Inorganic carbon adjustment models (Fontes and Garnier, 

1979; Tamers, 1975; Ingerson and Pearson, 1964; 

Mook, 1972; and Eichinger, 1983) determine an 

adjusted 14C activity (And) in the observed water with 

the assumption that radioactive decay of 14C has not 

occurred. The radiocarbon age of the water (∆t(years)) 

is determined by comparing the adjusted 14C activity to 

the measured 14C activity by means of the radioactive 

decay equation (Plummer and others, 1994).

τ T1 2⁄ 1n2 1

Hetrit

3

[ ]

H

3 = ⁄ × + --------------------

⎟

⎠

⎞ ⎜

⎝

⎛ ∆ = A

A

t years nd ln

ln 2

5,568 ( )

(1)

Figure 3. Atmospheric mixing ratios of chlorofluorocarbon-11 (CFC-11), chlorofluorocarbon-12 (CFC-12), 

chlorofluorocarbon-113 (CFC-113), and sulfur hexafluoride (SF6) for North American air andestimated monthly 

concentration of tritium (3H) in precipitation for Virginia. Tritium data derived from estimation technique of 

Michel (1989) and are not corrected for radioactive decay. Modified from Plummer and Busenberg (2000).

1940 1960 1980 2000

0

100

200

300

400

500

600

0

500

1,000

1,500

2,000

2,500

CFC-11

CFC-12

CFC-113

SF6 

X 100

3H

Bomb Peak

YEAR

CONCENTRATION IN AIR, IN 

PARTS PER TRILLION (VOLUME)

TRITIUM CONCENTRATION IN PRECIPITATION,

 IN TRITIUM UNITS

(2)
12 Aquifer Susceptibility in Virginia, 1998-2000

The adjusted radiocarbon ages are based on the 

Libby half-life of 5,568 years, reported in radiocarbon 

years BP (before present, where present is AD 1950), 

and were not calibrated to calendar years. The adjustment models were accessed through the geochemical 

model NETPATH (Plummer and others, 1994), and the 

following initial conditions were assumed: 

1. 14C activity in carbonate minerals of 0 percent 

modern carbon (pmc),

2. 14C activity in soil gas carbon dioxide (CO2) of 

100 pmc,

3. δ13C in carbonate minerals of 0 ‰ (per mil) 

relative to the Vienna Peedee belemnite 

(VPDB) standard, and

4. δ13C in soil gas CO2 of -20 ‰ relative to 

VPDB.

Acknowledgments

Christopher D. Adkins and Gerald W. Peaks of the 

VDH, Office of Drinking Water, provided valuable 

assistance with the public water-supply data base and 

project logistics. Field Directors and District Engineers 

from the regional field offices of the VDH provided 

contact and well-construction information. Assistance 

in the field sampling by USGS employees R.J. Ahlin, 

G.C. Casile, M.W. Doughten, K.M. Dydak, S.V. Harvey, H.M. Johnson, IV, W. Kirkland, R.M. Moberg, 

J.C. Puller, M.W. Strader, H.T. Tieu, and J.E. Wayland 

is gratefully appreciated. T.J. Burbey and W.J. Seaton 

of Virginia Polytechnic Institute and State University 

provided access to their research site and assisted with 

the field sampling. Ken Coffman and Steve Childers of 

the Virginia Rural Water Association and Terri Brown 

of the Virginia Department of Conservation and Recreation were of great assistance with providing contact 

and water-supply systems information and with sampling logistics. G.C. Casile, T.B. Coplen, M.W. Doughten, W. Kirkland, J.E. Wayland, and P. K. Widman of 

the USGS National Research Program in Reston, Va., 

provided valuable assistance with the preparation and 

delivery of sample equipment and supplies and performed the laboratory analyses. S.D. Shapiro of the 

USGS National Research Program in Reston, Va., provided data on detection of volatile organic compounds 

in the samples collected during this study. Special 

thanks are also expressed to the well owners and operators for providing access to their water supplies.

AQUIFER SUSCEPTIBILITY

The potential for an aquifer to be contaminated 

from near-surface sources is indicated by the presence 

of young waters. The VAS study used the CFC method 

(Busenberg and Plummer, 1992) as the principal indicator of the presence of young waters. Water samples 

from 171 wells and springs across Virginia were analyzed for concentrations of the CFC compounds 

trichlorofluoromethane (CCl3F, CFC-11, Freon 11), 

dichlorodifluoromethane (CCl2F2, CFC-12, Freon 12), 

and trichlorotrifluoroethane (C2Cl3F3, CFC-113), and 

for concentrations of 3H. The detection level for 

CFC-11 and CFC-12 was near 0.3 pg/kg (picogram per 

kilogram) and for CFC-113 was approximately 

1.0 pg/kg (Plummer and others, 1998). These detection 

levels are equivalent to less than 1 part per quadrillion 

(Plummer and others, 1998) and are about five orders 

of magnitude less than detection levels for standard 

drinking-water samples. Sorption and biodegradation 

processes can reduce CFC concentrations in ground 

water; therefore, 3H was used as a secondary indicator 

of the presence of young water. The detection level for 

3H was less than 0.3±0.3 TU (tritium unit). The results 

of CFC and 3H sampling of the 171 wells and springs 

were used to classify regional aquifer systems across 

Virginia as susceptible or not susceptible to contamination from near-surface sources. Apparent ages and the 

percentage of young water in the samples provided 

additional information on aquifer susceptibility. For 

samples that were indicative of binary mixtures of 

young and old (pre-CFC, or greater than 50 years) 

waters, CFC, SF6, and 3H/3He methods were used to 

estimate the apparent age of the young fraction. As a 

means of evaluating the validity of the VAS study 

methodology, the occurrence of selected chemical constituents in the water samples was compared to aquifer 

susceptibility determinations, apparent ground-water 

ages, and percentages of young water.
Aquifer Susceptibility 13

Determination of Susceptibility of Regional 

Aquifer Systems

Under the Commonwealth's SWAP, existing and 

potential threats to the quality of the public drinking 

water at each water system throughout Virginia are 

being evaluated by (1) delineating the source-water 

assessment area, (2) conducting a contaminant-source 

inventory, and (3) determining the susceptibility of the 

public water supply to contamination from the inventoried sources (Virginia Department of Health, 1999). 

Susceptibility determination is a three-step process 

(table 2). First, the findings from the VAS study will be 

used to determine if a ground-water source (community 

well or spring) is sensitive to land-use activities in the 

source-water assessment area. Second, the assessment 

area will be divided into two zones and land-use activities will be inventoried. A field inventory will be done 

in a 1,000-ft radius around the well (zone 1); in a 1-mi 

radius around the well (zone 2), potential sources of 

contamination will be identified from geographic information system (GIS) data layers from State and Federal 

regulatory authorities and other sources. Third, the 

SWAP susceptibility determination (very low, low, 

moderate, or high) will be assigned.

Results of the CFC and 3H analyses from the water 

samples from 171 wells and springs across Virginia 

were used to determine which regional aquifer systems 

are susceptible to contamination from near-surface 

sources. The VAS study used a threshold CFC concentration of 5 pg/kg as the principal indication of the 

presence of young waters (less than 50 years) in the 

regional aquifers sampled. The threshold CFC concentration is greater than the detection levels for the CFC 

compounds and was established to avoid possible complications introduced by analytical precision at 

extremely low levels of detection. The threshold was 

considered to be exceeded if one of the CFC concentrations was greater than the threshold value. A threshold 

concentration for 3H of 0.6 TU was used for samples 

that met at least one of the following criteria: (1) the 

threshold was not exceeded for any of the CFCs measured, (2) dissolved oxygen concentrations were low, 

(3) methane and hydrogen sulfide were detected, and 

(4) the aquifer was known to contain abundant organic 

matter (for example, coal and peat deposits). Sorption 

of CFCs to sediments and organic matter and biodegradation of the CFCs can decrease the concentrations of 

CFCs in ground water, making the water sample 

“appear” older than it actually is (Busenberg and Plummer, 1992; Plummer and Busenberg, 2000). In some 

situations, these processes can completely remove the 

CFCs from the water. For 3H, analytical precision at the 

detection level is ±0.3 TU; similar to the CFCs, the 

threshold 3H concentration is greater than the detection 

level plus the precision error concentration.

Summary statistics for averaged values from the 

replicate analyses for the CFCs and individual analyses 

of 3H concentrations for samples from the regional 

aquifer systems are provided in table 3. Concentrations 

that exceed the CFC and 3H thresholds indicate the 

presence of young waters (shaded cells in table 3). The 

large maximum concentrations for the CFCs indicate 

that sources other than the atmosphere have introduced 

CFCs into the ground water; therefore, precise age dating of these samples is not possible. However, these 

samples still indicate the presence of young waters. 

Threshold concentrations of at least one CFC compound and of 3H frequently were exceeded in the 

regional aquifer systems in the fractured-rock terrains 

and in the shallow parts of the Coastal Plain (fig. 4).

Appalachian Plateaus Regional Aquifer System

The threshold CFC concentration was exceeded in 

11 of the 13 wells (85 percent) sampled to represent the 

Appalachian Plateaus regional aquifer system (fig. 4). 

CFC concentrations ranged from less than 0.3 to 

4,113 pg/kg with median concentrations of 15.8, 13.4, 

and less than 1.0 pg/kg for CFC-11, CFC-12, and 

CFC-113, respectively (table 3). The threshold 3H conSensitive 

source1

Land-use

activity present

in source area

SWAP

susceptibility

No No Very Low

No Yes Low

Yes No Moderate

Yes Yes High

1The Virginia Department of Health will use the findings from the 

VAS study to determine if a ground-water source is sensitive to 

land-use activities in the source-water assessment area.

Table 2. Virginia Source Water Assessment Program 

(SWAP) susceptibility determinations for ground-water 

sources

[Modified from Virginia Department of Health, 1999]
14 Aquifer Susceptibility in Virginia, 1998-2000

centration was exceeded in all of the samples (fig. 4). 

The 3H concentrations ranged from 5.9 to 8.9 TU with 

a median concentration of 7.4 TU (table 3). Although 

the CFC concentrations in samples from wells AP-07 

and AP-11 were slightly less than the CFC threshold, 

the 3H concentrations from these two wells (7.3±0.1 

and 8.3±0.6 TU, respectively) were considerably 

greater than the 3H threshold. Because all of the water 

samples exceeded at least one of the threshold values, it 

is concluded that young water is present throughout 

most of the Appalachian Plateaus regional aquifer system. Water supplies developed in this aquifer system, 

therefore, are considered susceptible to contamination 

from near-surface sources (fig. 4).

Valley and Ridge Carbonate Regional Aquifer 

System

The threshold CFC concentration was exceeded in 

all of the 19 wells and 2 springs sampled to represent 

the Valley and Ridge Carbonate regional aquifer system (fig. 4). CFC concentrations ranged from less than 

0.3 to 129,128 pg/kg with median concentrations of 

311, 192, and 137 pg/kg for CFC-11, CFC-12, and 

CFC-113, respectively (table 3). The threshold 3H concentration was exceeded in all but one of the samples. 

The 3H concentrations ranged from 0.02 to 12.3 TU 

with a median concentration of 4.7 TU (table 3). Well 

VR-01, which is on the border between the Valley and 

Ridge and Blue Ridge Provinces, had a measured 3H 

concentration of 0.02±0.04 TU. This well is 632-ft 

deep with 500 ft of casing, which was grouted to 100 ft 

below land surface. Water enters this well from three 

zones between 526 and 578 ft, and the depth to bedrock 

is 494 ft. Although the CFC-11 concentration of 5.7 

pg/kg indicates that young water enters this well, the 

thick overburden and/or the manner in which the well 

is constructed may be limiting susceptibility to contamination from near-surface sources. Because all of the 

water samples exceeded at least one of the threshold 

values, it is concluded that young water is present 

throughout most of the Valley and Ridge Carbonate 

regional aquifer system. Water supplies developed in 

this aquifer system, therefore, are considered susceptible to contamination from near-surface sources (fig. 4).

Valley and Ridge Siliciclastic Regional Aquifer 

System

The threshold CFC concentration was exceeded in 

23 of the 24 wells and in the only spring (96 percent) 

sampled to represent the Valley and Ridge Siliciclastic 

regional aquifer system (fig. 4). CFC concentrations 

ranged from less than 1.0 to 5,350 pg/kg with median 

concentrations of 87.3, 65.8, and 26.4 pg/kg for 

CFC-11, CFC-12, and CFC-113, respectively (table 3). 

The threshold 3H concentration was met or exceeded in 

23 wells and 1 spring. The 3H concentrations ranged 

from 0.6 to 10.2 TU with a median concentration of 5.6 

TU (table 3). The concentrations of all three CFCs 

were below the thresholds in the sample from well

Aquifer

system

CFC-11

(pg/kg)

CFC-12

(pg/kg)

CFC-113

(pg/kg)

Tritium

(TU)

Min Max Med Min Max Med Min Max Med Min Max Med

AP 1.1 4,113.4 15.8 <0.3 3,244.6 13.4 <1.0 151.5 <1.0 5.9 8.9 7.4

VR-C 5.7 1,197.0 310.8 <.3 1,832.8 192.1 2 129,128.4 136.9 0.02 12.3 4.7

VR-S 1.3 846.9 87.3 3.1 5,349.9 65.8 <1.0 907.9 26.4 0.6 10.2 5.6

BR 9.4 9,853.1 134.7 69.1 98,347.9 299.8 <1.0 4,031.8 88.3 2.5 12.6 9

PD 2.7 19,410.5 253.7 8.1 9,472.1 232.3 <1.0 776.3 53.1 <.3 12.8 7.6

CP-S <.3 1,620.3 0.8 <.3 644.7 1.3 <1.0 634.3 <1.0 0.01 15.4 <.3

CP-M <.3 4.4 <.3 <.3 1.9 <.3 <1.0 0.4 <1.0 0.01 0.7 <.3

CP-D <.3 3.1 <.3 <.3 2.4 <.3 <1.0 <1.0 <1.0 <.3 0.6 <.3

Table 3. Summary statistics for average chlorofluorocarbon and tritium concentrations by regional aquifer system

[Shaded cells indicate values that exceed chlorofluorocarbon (CFC) and tritium thresholds of 5 picograms per 

kilogram (pg/kg) and 0.6 tritium unit (TU), respectively. Min, minimum; Max, maximum; Med, median; AP, 

Appalachian Plateaus; VR-C, Valley and Ridge Carbonate; VR-S, Valley and Ridge Siliciclastic; BR, Blue 

Ridge; PD, Piedmont; CP-S, Coastal Plain Shallow (depths less than 200 feet); CP-M, Coastal Plain Middle 

(depths between 200 and 400 feet); CP-D, Coastal Plain Deep (depths greater than 400 feet); <, less than]
Aquifer Susceptibility 15Figure 4. Exceedance of threshold chlorofluorocarbon and tritium concentrations and susceptibility determinations by regional aquifer system, Virginia.

REGIONAL AQUIFER SYSTEM

Appalachian

Plateaus

2

13

11

13

Valley and Ridge

Carbonate

21

1

20

21

Valley and Ridge

Siliciclastic

25

1

24 24

1

Blue Ridge

17 16 17

Piedmont

30

2

28

30

Coastal Plain

Shallow

18

16

23

11

20

14

Coastal Plain

Middle

19

1

19

1

20

Coastal Pl

Deep

11 11 

EXPLANATION

THRESHOLD CHLOROFLUOROCARBON CONCENTRATIONEXCEEDEDTHRESHOLD TRITIUM CONCENTRATION MET OR EXCEEDEDTHRESHOLD CHLOROFLUOROCARBON OR TRITIUMCONCENTRATION NOT EXCEEDED

23 Number of samples

SUSCEPTIBLE

SUSCEPTIBILITY DETERMINATION

NOT SUSCEPTIBLE

PERCENT

0

20406080

100
16 Aquifer Susceptibility in Virginia, 1998-2000

VR-35, which was drilled to 130 ft, cased and grouted 

to 60 ft, and encountered bedrock at 30 ft. This well 

produces water from three zones between 88 ft and the 

bottom of the well. A 3H concentration of 6.1±0.1 TU 

and a dissolved oxygen concentration of 0.5 mg/L were 

measured in well VR-35. Sorption and biodegradation 

processes possibly have reduced the CFC concentrations. The concentration of 3H (0.6±0.1) was slightly 

below the threshold value for the sample from well 

VR-11, which is downhill from well VR-01 and produces water from 7 zones between 333 and 490 ft. Well 

VR-11 was drilled to 490 ft, cased to 316 ft, grouted to 

100 ft, and encountered bedrock at 293 ft. The 

CFC-113 concentration (14.9 pg/kg) in the sample 

from well VR-11 indicates that young water is present. 

Review of the results from VR-01 and VR-11 suggests 

that the applicable age ranges of the various methods 

for age dating young ground water could be exceeded 

in hydrologic settings characterized by thick overburden (for example, the "Western Toe"). Despite the thick 

overburden and the manner of well construction, young 

waters were detected at the depth ranges for wells 

VR-01 and VR-11. Because all of the water samples 

exceeded at least one of the threshold values, it is concluded that young water is present throughout most of 

the Valley and Ridge Siliciclastic regional aquifer system. Water supplies developed in this aquifer system, 

therefore, are considered susceptible to contamination 

from near-surface sources (fig. 4).

Blue Ridge Regional Aquifer System

The threshold values for CFC and 3H concentrations were exceeded in all of the wells sampled to represent the Blue Ridge regional aquifer system (fig. 4). 

CFC concentrations ranged from less than 1.0 to 

98,348 pg/kg with median concentrations of 135, 300, 

and 88.3 pg/kg for CFC-11, CFC-12, and CFC-113, 

respectively (table 3). The 3H concentrations ranged 

from 2.5 to 12.6 TU with a median concentration of 9.0 

TU (table 3). Because all of the water samples 

exceeded at least one of the threshold values, it is concluded that young water is present throughout most of 

the Blue Ridge regional aquifer system. Water supplies 

developed in this aquifer system, therefore, are considered susceptible to contamination from near-surface 

sources (fig. 4).

Piedmont Regional Aquifer System

The threshold CFC concentration was exceeded in 

all of the 28 wells and 2 springs sampled to represent 

the Piedmont regional aquifer system (fig. 4). CFC 

concentrations ranged from less than 1.0 to 

19,410 pg/kg with median concentrations of 254, 232, 

and 53 pg/kg for CFC-11, CFC-12, and CFC-113, 

respectively (table 3). The threshold 3H concentration 

was met or exceeded in 26 wells and 2 springs (93 percent). The 3H concentrations ranged from 0.3 to 12.8 

TU with a median concentration of 7.6 TU (table 3). 

The 3H concentrations in the samples from wells 

PD-01 and PD-17 (0.3±0.03 and 0.6±0.3 TU, respectively) were below the threshold value. Well PD-01 was 

drilled as an irrigation well to a depth of 950 ft, cased 

to 101 ft, grouted to 21 ft, and encountered bedrock at 

90 ft. The well produces water from three zones 

between 200 and 950 ft. The driller reported that the 

lower water zone (940 to 950 ft) yields 200 gal/min 

(Robert Royall, Royall Pump and Well Co., Inc., written commun., 1999). The presence of CFCs in well 

PD-01 may be the result of natural ground-water flow 

or an artifact of the air-rotary drilling method because 

the well was completed less than a year prior to the 

date of sample collection. Recent research involving 

the addition of Halon 1211 (bromochlorodifluoromethane) into the drilling air indicates that the effects 

of the drilling were still evident in water samples collected 6 months after the date of well completion (L.N. 

Plummer, U.S. Geological Survey, written commun., 

2002). The 3H concentration in the water sample from 

well PD-17 was slightly below the threshold value, but 

this concentration could have been the result of analytical or sampling error, dilution of the young water with 

old (pre-CFC) waters, or matrix diffusion (Shapiro, 

2001). Because all of the water samples exceeded at 

least one of the threshold values, it is concluded that 

young water is present throughout most of the Piedmont regional aquifer system. Water supplies developed in this aquifer system, therefore, are considered 

susceptible to contamination from near-surface sources 

(fig. 4).

Coastal Plain Shallow Regional Aquifer System

The threshold CFC concentration was exceeded in 

13 of the 33 wells and in the only spring (41 percent) 

sampled to represent the Coastal Plain Shallow 
Aquifer Susceptibility 17

regional aquifer system (fig. 4). CFC concentrations 

ranged from less than 0.3 to 1,620 pg/kg with median 

concentrations of 0.8, 1.3, and less than 1.0 pg/kg for 

CFC-11, CFC-12, and CFC-113, respectively (table 3). 

The threshold 3H concentration was exceeded in 10 

wells and 1 spring (32 percent). The 3H concentrations 

ranged from 0.01 to 15.4 TU with a median concentration of less than 0.3 TU (table 3). The CFC and 3H concentrations measured in the samples from wells VB-07 

and VB-11 are thought to be an artifact of well drilling 

or the result of contamination during sample collection. 

Wells VB-07 and VB-11 are finished in very 

fine-grained sediments mapped as confining units by 

Smith and Harlow (2002). During sample collection, 

the yields of these wells were low (less than 1 gal/min) 

and the water levels were lowered to the pump setting, 

which was above the top of the screened interval. Samples were collected on the following day because of 

slow water-level recovery.

About 74 percent of the samples with the top of the 

first water zone less than 100 ft below land surface 

exceeded the susceptibility threshold values. The maximum depth to the top of the first water zone for which 

the threshold values were exceeded was 143 ft below 

land surface at well VB-09. These depths are similar to 

the depths at which young waters were detected by 

Nelms and others (2001) in a local study area in the 

Coastal Plain of Virginia. Young water was consistently 

detected in the first 100 ft of the Coastal Plain Shallow 

regional aquifer system. Consequently, the probability 

of water supplies being affected by near-surface 

sources of contamination is greatest in the upper 100 ft 

of the Coastal Plain Shallow regional aquifer system. 

Water supplies developed in the upper 100 ft of this 

aquifer system, therefore, are considered susceptible to 

contamination from near-surface sources. The analyses 

of the sample from spring CP-50 and results from previous studies of springs in the Coastal Plain of Virginia 

(Focazio and others, 1998; Nelms and others, 2001) 

indicate that springs in this regional aquifer system 

contain young water, and, therefore, are considered 

susceptible to contamination from near-surface 

sources.

Coastal Plain Middle Regional Aquifer System

The threshold CFC concentration was not equaled 

or exceeded in any of the 20 wells sampled to represent 

the Coastal Plain Middle regional aquifer system 

(fig. 4). CFC concentrations ranged from less than 0.3 

to 4.4 pg/kg with median concentrations of less than 

0.3 pg/kg for CFC-11 and CFC-12, and less than 

1.0 pg/kg for CFC-113 (table 3). The threshold 3H concentration was exceeded in only one well (5 percent). 

The 3H concentrations ranged from 0.01 to 0.7 TU with 

a median concentration of less than 0.3 TU (table 3). 

The low concentrations of CFCs and 3H may be the 

result of analytical or sampling error, or transient leakage from the well casing or wellbore. Because the 

threshold CFC concentration was not exceeded in any 

sample and a large percentage (95 percent) of the samples did not exceed the threshold 3H value, it is concluded that young water is generally not present 

throughout most of the Coastal Plain Middle regional 

aquifer system. Water supplies developed in this aquifer system, therefore, are considered not susceptible to 

contamination from near-surface sources (fig. 4).

Coastal Plain Deep Regional Aquifer System

The threshold concentrations for CFCs and 3H were 

not equaled or exceeded in any of the 10 wells sampled 

to represent the Coastal Plain Deep regional aquifer 

system (fig. 4). CFC concentrations ranged from less 

than 0.3 to 3.1 pg/kg with median concentrations of 

less than 0.3 pg/kg for CFC-11 and CFC-12, and less 

than 1.0 pg/kg for CFC-113 (table 3). The 3H concentrations ranged from less than 0.3 to 0.6 TU with a 

median concentration of less than 0.3 TU (table 3). 

Similar to the samples from the Coastal Plain Middle 

regional aquifer system, the low concentrations of 

CFCs and 3H may be the result of analytical or sampling error, or transient leakage from the well casing or 

wellbore. Because none of the water samples exceeded 

the threshold values, it is concluded that young water is 

not present throughout most of the Coastal Plain Deep 

regional aquifer system. Water supplies developed in 

this aquifer system, therefore, are considered not susceptible to contamination from near-surface sources 

(fig. 4).

Relation between Aquifer Susceptibility 

and Apparent Ground-Water Age

The multiple tracer approach allows for the determination of the apparent age of the sample and of 

whether the sample is a mixture of waters of different 
18 Aquifer Susceptibility in Virginia, 1998-2000

ages. Previous studies with multiple tracers (Plummer 

and others, 1999; Plummer and others, 2001) indicate 

that the analytical results from the CFC method can be 

interpreted using binary mixing models. The CFC data 

provide an indication of whether the sample represents 

a binary mixture from which the relative proportions of 

old (which is assumed to be pre-CFC) and young 

waters can be estimated. The apparent age of the young 

water fraction can be estimated from CFC ratios 

(CFC-11/CFC-12, CFC-113/CFC-11, and 

CFC-113/CFC-12) and/or from 3H/3He and SF6 dating 

methods. For example, the water sample from well 

PD-22 indicates that the apparent piston-flow ages, 

which assumes that the tracer (the CFC compound) is 

transported as a plug through the aquifer with no dispersive mixing in the direction of flow (Busenberg and 

Plummer, 1992), from CFC-11, CFC-12, and CFC-113 

are 22, 23, and 17 years, respectively (fig. 5). The 

apparent ages from CFC-113/CFC-12, 

CFC-113/CFC-11, and SF6 are 13, 14, and 15 years, 

respectively. The CFC ratios indicate that the percentage of the young fraction (15 years) in a binary mixture 

with old waters is between 64 and 72 percent (fig. 5). In 

most cases, the assumption that the old fraction represents pre-CFC waters is valid based on an evaluation of 

where the sample from well PD-22 plots in relation to 

the apparent ages of the young fraction from the CFC 

ratios, SF6, and 3H/3He (fig. 5). Various mixing models 

such as binary mixtures of only young waters or exponential models can be used to interpret the analytical 

results. These models also would indicate that the 

regional aquifer systems are susceptible, similar to the 

findings of the VAS study, although the degree of susceptibility may vary.

Young Waters

The apparent ages and percentages of young water, 

given the assumption of binary mixtures, are summarized in tables 5 and 6 (at end of report). The apparent 

ages of young water fractions statewide ranged from 

less than 1 to 48 years with a median apparent age of 

18 years (fig. 6). It is evident that the regional aquifer 

systems in the fractured-rock terrains and in the 

Coastal Plain Shallow system contain young water and 

therefore are susceptible to contamination from 

near-surface sources. The three samples containing 

young water from the Coastal Plain Middle and Deep 

regional aquifer systems probably represent analytical 

or sampling error or transient leakage from the well 

casing or wellbore. The dating methods for young 

waters were not applicable in 53 of the 171 samples 

because of (1) contamination of CFCs from sources 

other than the atmosphere, (2) sorption and degradation 

of CFCs, (3) contamination of SF6 and 3He from terrigenic sources, or (4) presence of paleowaters that do 

not contain the environmental tracers used to date 

young waters. More than 95 percent of the 118 samples 

for which the dating methods were applicable contained waters with apparent ages less than 35 years. 

About 5 percent of these samples contained young 

waters with apparent ages of less than 5 years, most of 

which were samples from the Blue Ridge and Piedmont regional aquifer systems. A relatively small percentage of the samples from the Valley and Ridge 

Carbonate regional aquifer system had apparent ages of 

less than 5 years.

Dye tracer studies are a common method for determining areas contributing recharge to springs and wells 

in carbonate terrains where travel times generally are 

on the order of days, weeks, or months. The multiple 

tracer approach and the assumption of binary mixtures 

of young and pre-CFC waters do not allow for dating 

waters with such rapid travel times. The apparent ages 

determined by the VAS study probably represent maximum ages of the young fractions because processes 

such as dilution and matrix diffusion (Shapiro, 2001) 

can alter the original tracer concentrations as the 

ground water moves through the flow system. The 

actual ages of the young fractions, therefore, may be 

younger than the apparent ages presented in this report 

and the regional aquifer systems may be more susceptible, especially in the fractured-rock terrains, than is 

indicated by this study.

In addition to apparent age, the amount of the 

young water fraction, assuming binary mixtures with 

old (pre-CFC) waters, provides information about aquifer susceptibility. The distribution of percent young 

water fractions from the binary mixture model is shown 

on figure 7. Most of the samples from the Valley and 

Ridge Carbonate, Blue Ridge, and Piedmont regional 

aquifer systems had greater than 50 percent young 

water fractions. Hydrogeologic characteristics, such as 

the karst features in the Valley and Ridge Carbonate 

and the steep hydraulic gradients, limited ground-water 

storage, and short flow paths (Stone and others, 1986) 

of the Blue Ridge and the Piedmont regional aquifer 

systems, may explain the abundance of young waters 

detected in these systems. Porous media flow, 
Aquifer Susceptibility 19

Figure 5. Mixing ratios of chlorofluorocarbon-113 (CFC-113) and chlorofluorocarbon-12 (CFC-12) (A) and CFC-113 and chlorofluorocarbon 

CFC-11 (B) for water from well PD-22, Lake Wilderness, Virginia, October 1999. The sample is compared to three hypothetical mixing models. The 

apparent recharge date of 1984 was determined by sulfur hexafluoride (SF6).

0 50 100 150 200 250 300

0

20

40

60

80

100

1990

2000

1970

1980

1950

1960

0 100 200 300 400 500 600

0

20

40

60

80

100

1950

1990

2000

1970

1980

1980

1984 (apparent recharge

date of young fraction)

1960

CFC-12, IN PARTS PER TRILLION BY VOLUME

CFC-113, IN PARTS PER TRILLION BY VOLUME

CFC-11, IN PARTS PER TRILLION BY VOLUME

CFC-113, IN PARTS PER TRILLION BY VOLUME

EXPLANATION 

PISTON FLOW

BINARY MIXTURE OF MODERN (2000) 

WATER WITH OLD (pre-CFC) WATER

BINARY MIXTURE OF YOUNG (1984) 

WATER WITH OLD (pre-CFC) WATER

WATER SAMPLE FROM WELL PD-22—

Error bars based on ±1 ̊C uncertainty 

in nitrogen-argon recharge temperature

APPARENT RECHARGE DATE

PERCENTAGE OF YOUNG FRACTION IN

BINARY MIXTURE

(A)

(B)

25%

50%

50%

75%

25%

50%

75%

1984 (apparent recharge

date of young fraction)
20 Aquifer Susceptibility in Virginia, 1998-2000

Figure 6. Distribution of apparent age of young water fraction in binary mixtures with old (pre-chlorofluorocarbon) waters by regional 

aquifer system, Virginia.

APPARENT AGE OF YOUNG WATER FRACTION, IN YEARS

0 10 20 30 40 50 60

EXPLANATION 

11

20

22

15

25

22

2

1

118

Circles indicate outliers when number of samples

exceeds 10 and individual samples when number

of samples does not exceed 10.

10 90 25 50 75 

PERCENTILE

11 (Number of samples)

REGIONAL AQUIFER SYSTEM

Appalachian

Plateaus

Valley and Ridge

Carbonate

Valley and Ridge

Siliciclastic

Blue Ridge

Piedmont

Statewide

Coastal Plain

Middle

Coastal Plain

Deep

Coastal Plain

Shallow

Figure 7. Distribution of young water fraction in binary mixtures with old (pre-chlorofluorocarbon) waters by regional aquifer system, Virginia.

PERCENTAGE OF YOUNG WATER FRACTION

REGIONAL AQUIFER SYSTEM

Appalachian

Plateaus

Valley and Ridge

Carbonate

Valley and Ridge

Siliciclastic

Blue Ridge

Piedmont

Statewide

Coastal Plain

Middle

Coastal Plain

Deep

Coastal Plain

Shallow

0 20 40 60 80 100

11

17

22

12

23

105

2

0

18

EXPLANATION 

Circles indicate outliers when number of samples

exceeds 10 and individual samples when number

of samples does not exceed 10.

10 90 25 50 75 

PERCENTILE

11 (Number of samples)
Aquifer Susceptibility 21

ground-water storage, and low hydraulic gradients in 

the Coastal Plain Shallow aquifer and low permeability 

of the rocks in the Appalachian Plateaus (Harlow and 

LeCain, 1991) are possible explanations for a majority 

of the water samples from these regional aquifer systems containing less than 40 percent young waters.

The percentage of the young water fraction, given 

the assumption of binary mixtures, generally decreases 

as the apparent age increases, as shown by the LOWESS (Helsel and Hirsch, 1992) line (fig. 8) for the 

water samples collected in this study. This relation is 

important because processes that reduce contaminant 

concentrations, such as biodegradation of organic compounds and denitrification of nitrate, can take place for 

a longer time in older waters. Additionally, contaminant concentrations can be reduced by dilution when 

small fractions of young waters containing contaminants mix with older, contaminant-free waters. Thus, 

ground waters with a large fraction (greater than 90 

percent) of extremely young water (less than 5 years) 

are highly susceptible to land-use activities within the 

area contributing recharge. Comparisons of the percentage of the young water fraction to factors other 

than apparent age, including casing and grout depth, 

depth to bedrock in the fractured-rocked terrains, and 

age of well, produced random distributions and no significant relations.

The regional aquifer systems can be used as a surrogate for geology in comparisons to apparent 

ground-water age and percentage of the young water 

fraction. Other than the general conclusions mentioned 

previously, no significant relations could be determined 

between geology and apparent age or percentage of 

young fraction (fig. 8). This is to be expected for water 

supplies developed in fractured-rock aquifers where the 

frequency, geometry, and interconnectedness of the 

fractures control the movement of ground water and 

may be more important than the lithology. Limited 

ground-water storage in these aquifers can cause 

hydraulic gradients to be steep when the aquifers are 

stressed by pumpage.

A group of samples from the Coastal Plain Shallow 

regional aquifer system has apparent ages that range 

from 20 to 40 years with generally less than 20 percent 

young water fractions (fig. 8). Most of these samples 

are from shallow confined aquifers overlain by one or 

two confining units. For samples from wells screened 

below a confining unit, the apparent age is at least 25 

years (fig. 9). Many of the samples from the shallow 

confined aquifers did not exceed the susceptibility 

threshold values, but these samples still could be dated 

by the multiple tracer approach (fig. 9). The presence 

of young waters beneath 30 to 60 ft of confining units 

is consistent with the concept of leaky or breached confining units (Powell and others, 1990; Brockman and 

others, 1997; Nelms and others, 2001). Under stressed 

conditions, the head in the shallow confined aquifer 

may be lowered and cause the downward gradient to 

increase, which can induce flow from the overlying 

unconfined (water table) aquifer.

About 32 percent of the samples contained CFCs in 

excess of modern air (1998 to 2000) equilibrium. 

These large CFC mixing ratios indicate that the local 

air is enriched with CFCs or that a nonatmospheric 

source has contributed to CFCs measured in the water 

samples. Szabo and others (1996) estimated that local 

air in a southern New Jersey study area was enriched 

with CFCs by about 10 to 30 percent. In contrast, the 

CFCs measured during the VAS study generally were 

orders of magnitude greater than modern air mixing 

ratios, suggesting that sources other than the atmosphere (for example, sewage) have introduced CFCs 

into the ground water. The presence of CFC contamination (where “contamination” refers to an effect on 

ground-water dating, not to exceedance of water-quality standards) is another means of assessing the validity 

of the VAS methodology for determining aquifer susceptibility to near-surface contamination.

One or more of the CFCs were the result of contamination in 48 percent of the water samples from the 

fractured-rock terrains and in 6 percent of the samples 

from the Coastal Plain regional aquifer systems, all of 

which were from the Coastal Plain Shallow regional 

aquifer system (fig. 10). None of the samples from the 

Coastal Plain Middle and Deep regional aquifer systems contained CFC contamination, and CFCs were 

not detected in a majority of the samples. The high percentage of samples with measured CFC contamination 

in the fractured-rock terrains indicates that supplies 

developed in these regional aquifer systems are susceptible. The absence of either CFCs or CFC contamination in the samples from the Coastal Plain Middle and 

Deep regional aquifer systems illustrates the lack of 

connection and the long period of time it takes water to 

flow to these aquifers from near land surface under natural conditions.
22 Aquifer Susceptibility in Virginia, 1998-2000

Figure 8. Percentage of young water in relation to apparent age of young water fraction in binary mixtures with old 

(pre-chlorofluorocarbon) waters by regional aquifer system, Virginia.

0 10 20 30 40 50 60

0

20

40

60

80

100

APPARENT AGE OF YOUNG WATER FRACTION, IN YEARS

PERCENT OF YOUNG WATER

EXPLANATION 

Appalachian Plateaus

REGIONAL AQUIFER SYSTEM

Valley and Ridge Carbonate

Valley and Ridge Siliciclastic

Blue Ridge

Piedmont

Coastal Plain Shallow

Coastal Plain Middle

LOWESS LINE

Figure 9. Total thickness of the confining units in relation to apparent age of young water fraction in binary mixtures with old 

(pre-chlorofluorocarbon) waters for samples from the Coastal Plain Shallow regional aquifer system, Virginia.

APPARENT AGE OF YOUNG WATER FRACTION, IN YEARS

EXPLANATION 

TOTAL THICKNESS OF CONFINING UNITS,

IN FEET

Unconfined aquifer sample that exceeded

susceptibility threshold values

Unconfined aquifer sample that did not

exceed susceptibility threshold values

0 10 20 30 40 50 60

0

10

20

30

40

50

60

70

Confined aquifer sample that exceeded

susceptibility threshold values

Confined aquifer sample that did not

exceed susceptibility threshold values
Aquifer Susceptibility 23

Figure 10. Distribution of chlorofluorocarbon (CFC) contamination by regional aquifer system, Virginia.

Appalachian Plateaus Valley and Ridge Carbonate

Valley and Ridge Siliciclastic Blue Ridge

Piedmont Coastal Plain Shallow

Coastal Plain Middle Coastal Plain Deep

NUMBER OF SAMPLES

TYPE OF CFC CONTAMINATION

None

CFC-11

CFC-12

CFC-11 and

CFC-12

CFC-113

CFC-12 and

CFC-113

All CFCs

Total with CFC

contamination

None

CFC-11

CFC-12

CFC-11 and

CFC-12

CFC-113

CFC-12 and

CFC-113

All CFCs

Total with CFC

contamination

None

CFC-11

CFC-12

CFC-11 and

CFC-12

CFC-113

CFC-12 and

CFC-113

All CFCs

Total with CFC

contamination

None

CFC-11

CFC-12

CFC-11 and

CFC-12

CFC-113

CFC-12 and

CFC-113

All CFCs

Total with CFC

contamination

0 10 20 30 0 10 20 30
24 Aquifer Susceptibility in Virginia, 1998-2000

Old Waters or Paleowaters

The presence of old waters (pre-CFC or greater 

than 50 years) or paleowaters (greater than 1,000 years) 

in water samples was used as an indication that these 

regional aquifer systems probably are not affected by 

near-surface activities and, therefore, are not susceptible to contamination under natural conditions. Radiocarbon (carbon-14 or 14C) dating (Plummer and others, 

1994; Clark and Fritz, 1997; Kalin, 2000) was conducted on 34 water samples containing old or paleowaters from the Coastal Plain regional aquifer systems. 

The radiocarbon ages summarized in table 5 are 

adjusted radiocarbon ages based on the Fontes and Garnier (1979) model. The uncertainties in the radiocarbon 

age estimates were determined by varying by ±5‰ the 

δ13C in soil gas CO2 input for the Fontes and Garnier 

(1979) model. The distribution of adjusted radiocarbon 

ages for water samples from the Coastal Plain regional 

aquifer systems is summarized in table 4. The negative 

value for the minimum adjusted radiocarbon age in the 

Coastal Plain Shallow regional aquifer system is 

explained by the presence of young water for which the 

initial 14C activity was greater than 100 pmc as a result 

of atmospheric thermonuclear weapons testing and 

nuclear power plant releases (Clark and Fritz, 1997). 

Radiocarbon dating is therefore not viable and was not 

done for samples containing young waters, which 

included samples from the fractured-rock terrains and 

some samples from the Coastal Plain Shallow regional 

aquifer system.

The presence of paleowaters in the Coastal Plain 

regional aquifer systems is consistent with conceptual 

and flow-simulation models of this region (Harsh and 

Laczniak, 1990; Laczniak and Meng, 1988; Hamilton 

and Larson, 1988; McFarland, 1998). In these models, 

ground-water recharge occurs along the Fall Line, and 

water flows towards the Atlantic Ocean along deep 

regional flow paths. Adjusted radiocarbon ages of 

water samples increase as distance from the Fall Line 

increases (fig. 11); the samples from wells CP-11 and 

CP-21 in the Coastal Plain Shallow regional aquifer 

system, however, represent relatively local recharge 

and flow along local or subregional flow paths where 

there is potential for encountering young water (Nelms 

and others, 2001).

The relation between increasing adjusted radiocarbon ages and distance from the Fall Line for samples 

from the Cretaceous aquifers (upper, middle, and lower 

Potomac aquifers) appears to differ with regard to orientation of a given site with respect to the James River. 

Samples from north of the James River become older 

over a shorter distance from the Fall Line than those 

samples from south of the James River (fig. 11). This 

apparent relation is more prevalent in the samples from 

the middle Potomac aquifer, except for wells CP-24, 

CP-27, and CP-51. Samples from these three wells also 

have measured concentrations of sodium and chloride 

that are large relative to the other samples from the 

Coastal Plain aquifers. These wells are located in the 

hydrogeochemical feature that Cederstrom (1943, 

1946a, 1946b) referred to as the “inland saltwater 

wedge.” This difference in the progression of adjusted 

radiocarbon ages in the Cretaceous aquifers may result 

from the bifurcation of flow, as proposed by Cederstrom (1943, 1946a, 1946b), around what is now 

known to be the Chesapeake Bay impact crater (Powars 

and Bruce, 1999; Powars, 2000).

The adjusted radiocarbon ages also increase as the 

total thickness of the overlying confining units increase 

(fig. 12). It is also true that as the distance from the Fall 

Line increases, the thickness of the aquifers and confining units increases. The degree of confinement from 

near-surface activities is greater in the Coastal Plain 

Middle and Deep regional aquifer systems, where confining units are buried under a greater accumulation of 

sediments than in the Coastal Plain Shallow regional 

aquifer system (fig. 9). Conjunctively, the relations 

between adjusted radiocarbon ages and (1) distance 

from the Fall Line and (2) thickness of the overlying 

confining units indicate that the combination of long

Statistic

Coastal Plain regional

aquifer system

Shallow Middle Deep

Minimum -1,200 9,090 27,900

Median 2,400 24,200 33,600

Maximum 10,100 36,900 39,700

Number of samples 4 20 10

Table 4. Summary statistics for adjusted Fontes-Garnier 

radiocarbon ages (Fontes and Garnier, 1979) in the Coastal 

Plain regional aquifer systems

[Adjusted radiocarbon ages are in radiocarbon years BP 

(before present where present is AD 1950) based on the 

Libby half-life of 5,568 years and have not been 

calibrated to calendar years]
Aquifer Susceptibility 25

Figure 11. Relation between Fontes-Garnier adjusted radiocarbon age (Fontes and Garnier, 1979) and distance from the Fall Line 

by Coastal Plain regional aquifer system and aquifer. See figure 1 for location of Fall Line in Virginia.

0 20 40 60 80 100

0

10,000

20,000

30,000

40,000

50,000

CP- 39

CP- 35

CP- 22

CP- 19

CP- 31

CP- 47

CP- 29

CP- 37

CP- 32

CP- 03 CP- 40

CP- 36

CP- 30

CP- 01

CP- 24

CP- 20

CP- 34

CP- 27

CP- 51

CP- 18

CP- 42

CP- 06

CP- 04

CP- 21

CP- 11

CP- 23

CP- 25

CP- 02 CP- 07 CP- 10

CP- 08

CP- 05

CP- 41

DISTANCE FROM FALL LINE, IN MILES

FONTES-GARNIER ADJUSTED RADIOCARBON AGE,

IN RADIOCARBON YEARS BEFORE PRESENT

EXPLANATION 

REGIONAL AQUIFER SYSTEM

AQUIFER

Coastal Plain Shallow

Coastal Plain Middle

Coastal Plain Deep

Columbia

Yorktown-Eastover

Chickahominy-Piney Point

Aquia

Upper Potomac

Middle Potomac

Lower Potomac

Well located south of 

the James River

Figure 12. Relation between total thickness of confining units and Fontes-Garnier adjusted radiocarbon age by 

Coastal Plain regional aquifer system and aquifer, Virginia.

0 10,000 20,000 30,000 40,000 50,000

0

100

200

300

400

500

FONTES-GARNIER ADJUSTED RADIOCARBON AGE,

IN RADIOCARBON YEARS BEFORE PRESENT

TOTAL THICKNESS OF CONFINING

UNITS, IN FEET

EXPLANATION 

REGIONAL AQUIFER SYSTEM

AQUIFER

Coastal Plain Shallow

Coastal Plain Middle

Coastal Plain Deep

Columbia

Yorktown-Eastover

Chickahominy-Piney Point

Aquia

Upper Potomac

Middle Potomac

Lower Potomac
26 Aquifer Susceptibility in Virginia, 1998-2000

flow paths and slow flow through low-permeability 

confining units results in long ground-water residence 

times. This is the first quantification of a common conceptual model for the Coastal Plain aquifer systems in 

Virginia.

Relation between Aquifer Susceptibility 

and Selected Chemical Constituents

Comparison of susceptibility and apparent age 

determinations to the occurrence of selected chemical 

constituents in ground water is an additional means of 

evaluating the validity of the VAS study methodology. 

Nitrite plus nitrate as nitrogen (NO2+NO3-N), which is 

essentially nitrate because of low nitrite concentrations, 

is introduced into ground-water systems from commercial fertilizers, manure, and sewage effluent. Volatile 

organic compounds (VOCs) have been produced in 

considerable quantities and have widespread use in the 

United States (Squillace, Moran, Lapham, and others, 

1999). Because of these factors, VOCs are eventually 

released into the environment and are frequently 

detected in ground water (Westrick and others, 1984; 

U.S. Environmental Protection Agency, 1997; Squillace, Moran, Lapham, and others, 1999; Clawges and 

others, 2001; Grady and Casey, 2001; Moran and others, 2001; Stackelberg and others, 2001; Shapiro and 

others, 2003). The occurrence and distribution of 

nitrate and VOCs are indicators of contamination from 

near-surface sources and, thus, of aquifer susceptibility 

and vulnerability to contamination (Rupert, 1998; Shelton and others, 2001).

Nitrate

Nitrate (NO3) concentrations were measured in 

water samples from 171 wells and springs during the 

VAS study between 1998 and 2000 (Nelms and Harlow, 2003). The NO3 concentrations did not exceed the 

U.S. Environmental Protection Agency's Maximum 

Contaminant Level (MCL) of 10 mg/L in any samples 

and ranged from less than the USGS detection levels 

(0.005 mg/L and 0.05 mg/L) to 9.92 mg/L. Nitrate concentrations measured in the samples increase as the 

CFC mixing ratios increase, which means that NO3

concentrations are higher in younger waters (fig. 13). 

Generally, the highest NO3 concentrations are in waters 

with CFC mixing ratios that were greater than ratios in 

equilibrium with modern air for the year 2000, which is 

considered contaminated for dating purposes. There are 

three possible explanations for the small NO3 concentrations (equal to or slightly greater than the USGS 

detection levels) associated with CFC contamination: 

(1) the NO3 in ground water has been removed by denitrification processes; (2) the NO3 concentration has 

been reduced by dilution; or (3) a source for NO3 was 

not within the area contributing recharge to the well or 

spring sampled. These three scenarios illustrate the 

limitations of aquifer susceptibility methods based on 

the occurrence and distribution of NO3 in ground water. 

In terms of apparent age of the young water fraction, 

the number of samples with NO3 concentrations greater 

than the USGS detection levels generally increases as 

apparent age of the young fraction decreases (fig. 14a). 

Nitrate concentrations were equal to or slightly greater 

than the USGS detection levels for samples that contained only old or paleowaters (fig. 14b).

On the basis of the NO3 concentrations in the old or 

paleowaters, the aquifer susceptibility determinations 

were evaluated by comparing the NO3 concentrations 

to the USGS detection level of 0.05 mg/L. About 43 

percent of the samples were greater than the detection 

level (fig. 15). Most of these samples were from the 

fractured-rock terrains, especially the Valley and Ridge 

Carbonate regional aquifer system, where 90 percent of 

the samples had greater than 0.05 mg/L of NO3. The 

reporting limit was exceeded in 57 and 65 percent of 

the samples from the Piedmont and Blue Ridge 

regional aquifer systems, respectively. Although the 

evaluation of NO3 concentrations and the multiple 

tracer approach yielded somewhat consistent results in 

terms of aquifer susceptibility, the viability of NO3

concentrations as an indicator of aquifer susceptibility 

in the Appalachian Plateaus, Valley and Ridge 

Siliciclastic, and Coastal Plain Shallow regional aquifer 

systems can be limited by processes such as dilution 

and methanogenesis, and by proximity to near-surface 

sources.

Volatile Organic Compounds

VOCs are synthetic organic chemicals that are used 

in adhesives, dry cleaning, paints, plastics, solvents, 

and petroleum products. Many VOCs are mobile, persistent, and toxic compounds that are frequently 

detected in ground water (Squillace, Moran, Lapham, 

and others, 1999; Clawges and others, 2001; Grady and
Aquifer Susceptibility 27

Figure 13. Nitrate concentrations in relation to chlorofluorocarbon (CFC) mixing ratios in samples from 171 wells and springs 

in Virginia.

0.01

0.1

1

10

0.1 1 10 100 1,000 10,000

CFC-11, IN PARTS PER TRILLION (VOLUME)

1 10 100 1,000 10,000 100,000 1,000,000

0.01

0.1

1

10

CFC-12, IN PARTS PER TRILLION (VOLUME)

0.1 1 10 100 1,000 10,000 100,000

0.01

0.1

1

10

CFC-113, IN PARTS PER TRILLION (VOLUME)

NITRATE CONCENTRATION, IN MILLIGRAMS

PER LITER AS NITROGEN

Appalachian Plateaus

Valley and Ridge Carbonate

Valley and Ridge Siliciclastic

Blue Ridge

Piedmont

Coastal Plain Shallow

Coastal Plain Middle

Coastal Plain Deep

U.S. ENVIRONMENTAL PROTECTION

AGENCY MAXIMUM CONTAMINANT

LEVEL FOR NITRATE

U.S. GEOLOGICAL SURVEY NITRATE

DETECTION LEVEL—Analyses

of samples collected for the Virginia 

Aquifer Susceptibility study

U.S. GEOLOGICAL SURVEY NITRATE

DETECTION LEVEL—Analyses

of samples collected for the City of 

Virginia Beach shallow aquifer 

system study (Johnson, 1999)

REGIONAL AQUIFER SYSTEM

EXPLANATION

CFC MODERN (2000) ATMOSPHERIC 

MIXING RATIO

CFC contamination

CFC contamination

CFC contamination
28 Aquifer Susceptibility in Virginia, 1998-2000

Figure 14. Nitrate concentrations in relation to apparent ages of young water fractions in binary mixtures with old (pre-chlorofluorocarbon) 

waters (A) and to Fontes-Garnier adjusted radiocarbon ages (B), Virginia.

0 10 20 30 40 50 60

0.001

0.01

0.1

1

10

0 10,000 20,000 30,000 40,000 50,000

0.001

0.01

0.1

1

10

APPARENT AGE OF YOUNG WATER FRACTION, IN YEARS

ADJUSTED RADIOCARBON AGE, IN RADIOCARBON 

YEARS BEFORE PRESENT

NITRATE CONCENTRATION, IN MILLIGRAMS 

PER LITER AS NITROGEN

(A)

(B)

Appalachian Plateaus

Valley and Ridge Carbonate

Valley and Ridge Siliciclastic

Blue Ridge

Piedmont

Coastal Plain Shallow

Coastal Plain Middle

Coastal Plain Deep

U.S. ENVIRONMENTAL PROTECTION

AGENCY MAXIMUM CONTAMINANT

LEVEL FOR NITRATE

U.S. GEOLOGICAL SURVEY NITRATE

DETECTION LEVEL—Analyses

of samples collected for the Virginia 

Aquifer Susceptibility study

U.S. GEOLOGICAL SURVEY NITRATE

DETECTION LEVEL—Analyses

of samples collected for the City of 

Virginia Beach shallow aquifer 

system study (Johnson, 1999)

REGIONAL AQUIFER SYSTEM

EXPLANATION
Aquifer Susceptibility 29

Casey, 2001; Moran and others, 2001; Stackelberg and 

others, 2001). The production of these compounds in 

the United States increased by an order of magnitude 

between 1945 and 1985 (Ashford and Miller, 1998). 

The fuel oxygenate methyl tert-butyl ether (MTBE) 

was the 39th-highest organic chemical produced in 

1970, and by 1998 was 4th-highest in the United States 

(Squillace, Moran, and Clawges, 1999; Johnson and 

others, 2000). The atmospheric increase of the environmental tracers used during the VAS study coincides and 

to some degree mimics the use and production of 

VOCs in the United States. Shapiro and others (2003) 

used archived chromatograms and water samples collected between 1989 and 2000 from more than 2,600 

sites throughout the United States to assess low-level 

(parts per quadrillion) detections of halogenated VOCs. 

These samples were originally analyzed by gas chromatography with an electron capture detector 

(GC-ECD) in the USGS CFC laboratory in Reston, Va., 

for the purpose of ground-water dating. About 6 percent of these analyses were from water samples collected for the VAS study.

The run times on the GC-ECD are sufficient to 

detect halogenated VOCs other than CFCs in water 

samples. The identification of the VOCs is only qualitative because quantitative analysis to determine actual 

concentrations is not possible with GC-ECD. However, 

Shapiro and others (2003) state that low-level detections of VOCs by GC-ECD can be used to indicate 

aquifer susceptibility and provide early warning of 

potential contamination of ground-water sources. Comparison of GC-ECD chromatograms for water samples 

from the different regional aquifer systems is another 

means of validating the VAS study methodology. For 

example, the chromatogram shows that water from well 

CP-08 produces paleowater (about 34,000 radiocarbon 

years) that has not been affected by VOCs (fig. 16), and 

thus probably is not affected by anthropogenic activities. The chromatogram for spring SNP-028 indicates 

that this water is young and contains CFCs and VOCs 

that are probably of atmospheric origin. Contamination 

from near-surface sources is illustrated by the chromatograms for wells CP-15 and VR-23, which contain 

several VOC peaks with large peak areas (fig. 16). Furthermore, these two wells illustrate the disparity with 

relation to depth of contaminant migration between 

supplies developed in the unconsolidated deposits of 

the Coastal Plain and in the fractured-rock terrains 

where contaminants can readily migrate to relatively 

deep depths. 

One or more VOC peaks were frequently identified 

from the GC-ECD chromatograms for the samples

Figure 15. Distribution by regional aquifer system (in Virginia) of nitrate concentrations greater than the 

U.S. Geological Survey (USGS) detection level of 0.05 milligrams per liter. Numbers in italics indicate 

percentage of wells that exceeded the USGS detection level.

NUMBER OF SAMPLES

15

90

40

65

57

32

10

18

43

0 10 20 30 40 50 60 70

REGIONAL AQUIFER SYSTEM

Appalachian

Plateaus

Valley and Ridge

Carbonate

Valley and Ridge

Siliciclastic

Blue Ridge

Piedmont

Statewide

Coastal Plain

Middle

Coastal Plain

Deep

Coastal Plain

Shallow
30 Aquifer Susceptibility in Virginia, 1998-2000

Figure 16. Chromatograms for selected wells and a spring in Virginia from gas chromatography with an electron capture detector (GC-ECD) for chlorofluorocarbon (CFPlummer and others (2001).

Spring SNP-028Aquifer system: Blue RidgeYoung, clean water

CFC-12

CFC-11

CFC-113

CCl4

Well CP-15Aquifer system: Coastal Plain ShallowDepth: 59 feetWater Zone: 39-44 feetYoung, contaminated water

CFC-12

CFC-11

CFC-113

Chloroform?low parts per billion (ppb)

PCE?ppb range

DCE?, TCA?, CH3Br?

CCl4

N2O

Well VR-23Aquifer systeDepth: 625 feWater Zone: Young, conta

GC-ECD RELATIVE RETENTION TIME, IN MINUTES

RELATIVE CONCENTRATION

48

1216

Well CP-08Aquifer system: Coastal Plain MiddleDepth: 448 feet

Water Zone: 412-438 feet

Paleowater

N2O

DCETCACH3Br 

CCl4

PCE

Nitrous oxide1,1-dichloroethylene1,1,1-trichloroethaneBromomethaneCarbon tetrachlorideTetrachloroethylene

EXPLANATION 
Aquifer Susceptibility 31

from the regional aquifer systems in the fractured-rock 

terrains and the shallow parts of the Coastal Plain 

(fig. 17). Shapiro and others (2003) classified the VOC 

peak areas into three groups based on the number of 

counts recorded by the detector: small (100,000 to 

499,000 counts), medium (500,000 to 999,999 counts), 

and large (greater than 1,000,000 counts). The range in 

peak area counts approximately corresponds to VOC 

concentrations from parts per quadrillion to parts per 

billion (Shapiro and others, 2003). Numerous VOCs 

with small to large peak areas were detected in the samples from the regional aquifer systems in the fractured-rock terrains and the Coastal Plain Shallow 

regional aquifer system (fig. 17a). The presence of 

VOCs in these samples is to be expected because the 

susceptibility threshold concentrations were exceeded 

and the regional aquifer systems are classified as susceptible to contamination from near-surface activities. 

The frequent detections of carbon tetrachloride and 

chloroform are in response to the high sensitivity of the 

GC-ECD to these compounds (Shapiro and others, 

2003). The detection of 1,1,1-trichloroethane (TCA), 

trichloroethylene (TCE), and tetrachloroethylene 

(PCE) in the water samples is consistent with the widespread and abundant use of these compounds (Harte 

and others, 1991); TCE and PCE detections are of concern because these compounds are known carcinogens 

and eventually degrade to vinyl chloride, which is a 

more toxic compound (Harte and others, 1991).

The identification of VOC peaks in the samples 

from the Coastal Plain Middle and Deep regional aquifer systems is problematic (fig. 17b). Generally, the 

VOC peaks identified in the samples from the Coastal 

Plain Middle and Deep regional aquifer systems are 

classified as small peak areas and usually correspond to 

the retention times for carbon tetrachloride (fig. 17b). 

The samples with medium to large peak areas for carbon tetrachloride typically had low detections of 3H 

(0.4 to 0.6 TU) and were from wells that were drilled 

prior to 1973. The combination of these factors suggests that transient leakage from the well casing or 

wellbore is allowing young water to enter the well. 

Another possibility is that the VOCs are remnants from 

well-construction or pump-maintenance activities.

Comparison of the number of VOCs detected and 

the major land-use category within 1,000 ft of the well 

or spring suggests that the occurrence of VOCs is only 

slightly greater in the developed areas than in forested 

and agricultural areas (fig. 18). However, most of the 

samples were collected from public supplies located in 

or near developed areas. The occurrence of VOCs is 

probably related to the proximity of the well or spring 

to specific local anthropogenic activities, not the major 

land-use activity within 1,000 ft of the well. Furthermore, Franke and others (1998) show that the area contributing recharge to a well does not have to include the 

location of the well or the area encompassing a specified distance from the well.

Statewide Assessment of Aquifer 

Susceptibility

Susceptibility of the regional aquifers in Virginia to 

contamination from near-surface sources depends on a 

complex array of natural and anthropogenic factors. 

The multiple tracer approach was used during the VAS 

study to measure the relation between these factors, 

and the results of the study can be summarized in terms 

of the type and relative degree of susceptibility of the 

regional aquifer systems (fig. 19). There are two types 

of regional aquifer systems: porous media (unconsolidated deposits) aquifers in the Coastal Plain regional 

aquifer systems, and fractured-rock and karst aquifers 

in the Appalachian Plateaus, Valley and Ridge, Blue 

Ridge, and Piedmont regional aquifer systems. The relative degree of susceptibility is determined on the basis 

of the amount and apparent age of the young water 

fraction in binary mixtures.

In the Coastal Plain, aquifer susceptibility depends 

upon the depth and degree of confinement of the aquifers and the type of flow paths intercepted. Most of the 

water samples from the Coastal Plain regional aquifer 

systems did not exceed the susceptibility threshold concentrations for CFCs and 3H (fig. 19). The frequency of 

NO3 detections in the water samples from the Coastal 

Plain Shallow regional aquifer system was greater than 

in those from the Coastal Plain Middle and Deep 

regional aquifer systems. Although the frequency of 

detection of VOCs in the samples from the Coastal 

Plain Middle and Deep regional aquifer systems was 

high, these detections are thought to be the result of 

analytical or sampling error, transient leakage from the 

well casing or wellbore, or remnants of well-construction or pump maintenance activities (fig. 19). Young 

water was consistently detected in the samples from the 

water table and shallow confined aquifers in the upper 

100 ft of the Coastal Plain Shallow regional aquifer 

system (fig. 20). Although the connection to near-sur-
32 Aquifer Susceptibility in Virginia, 1998-2000

Figure 17. Peak areas in relation to retention times for volatile organic compounds (VOCs) from gas chromatography with an 

electron capture detector (GC-ECD) analysis for samples that exceeded susceptibility threshold concentrations (A) and for 

samples that did not exceed susceptibility threshold concentrations (B). Data from Shapiro and others (2003).

0246 8 10 12 14 16 18 20

0246 8 10 12 14 16 18 20

105

106

107

108

109

105

106

107

108

109

DCE

TCA

CCl4

TCE

PCE

1,1-dichloroethylene

1,1,1-trichloroethane

Carbon tetrachloride

Trichloroethylene

Tetrachloroethylene

CCl4

DCE TCA Chloroform TCE PCE

CCl4

DCE TCA Chloroform TCE PCE

Large

Medium

Small

Large

Medium

Small

(A)

(B)

GC-ECD RETENTION TIME, IN MINUTES

V0Cs PEAK AREA, IN COUNTS

Appalachian Plateaus

Valley and Ridge Carbonate

Valley and Ridge Siliciclastic

Blue Ridge

Piedmont

Coastal Plain Shallow

Coastal Plain Middle

Coastal Plain Deep

REGIONAL AQUIFER SYSTEM

VOLATILE ORGANIC COMPOUND

EXPLANATION
Aquifer Susceptibility 33

face sources is limited for the shallow confined aquifers, the presence of young waters is facilitated by the 

thin and leaky nature of the overlying confining units 

and mixtures of waters that flow along local to subregional flowpaths. The confined aquifers of the Coastal 

Plain Middle and Deep regional aquifer systems are 

overlain by a series of relatively thick confining units 

and contain old (pre-CFC) or paleowaters from subregional to regional flow paths. The water samples from 

these aquifer systems rarely exceed the susceptibility 

thresholds (fig. 20), which indicates that the susceptibility of these aquifers under natural conditions is negligible because the radiocarbon ages of these waters are 

usually greater than 1,000 years. However, the potential 

of introducing young waters from well integrity problems exists, as evidenced by the VOC detections in 

some of the water samples from these regional aquifer 

systems.

The fractured-rock and karst aquifers of the Appalachian Plateaus, Valley and Ridge, Blue Ridge, and 

Piedmont regional aquifer systems are susceptible to 

contamination from near-surface sources for the depth 

ranges investigated during the VAS study (fig. 19). All 

of the water samples from the fractured-rock terrains 

are classified as susceptible, and no relation exists 

between susceptibility and depth of the top of the first 

water zone or well depth (fig. 20). Ground-water movement along fractures, joints, and solution features can 

be rapid, especially under stressed conditions that can 

result from pumpage. The geometry of these features 

can allow young ground water to circulate to substantial depths in these regional aquifer systems, and the 

depth at which the water enters the well (water zone) is 

more important than the total depth of the well 

(fig. 20). Previous investigations have identified the 

presence of young waters in these aquifer systems 

(Nelms and Ahlin, 1993; Nelms and Brockman, 1997; 

Focazio and others, 1998; Nelms and others, 1999; 

Kozar and others, 2001; Plummer and others, 2001). 

Similar findings from environmental tracer studies in 

South Carolina have also identified the presence of 

young waters in the Piedmont and Blue Ridge (Stone 

and others, 1986; Stone and others, 1989; Stone and 

others, 1992; Stone and others, 1997; Stone and Devlin, 1999a, 1999b). The frequent detections of NO3 and 

VOCs, and CFC concentrations in excess of air-water 

equilibrium in water samples further support the classification of these aquifer systems as susceptible. 

Numerous investigations have documented the frequent 

occurrence of ambient flow within open boreholes in 

fractured-rock terrains (Williams and others, 2002; 

Johnson and others, 2002; Heisig, 2002). This mechanism in conjunction with rapid movement along fractures, joints, and solution features may be responsible 

for the ubiquitous occurrence of some fraction of 

young waters at depth in the fractured-rock regional 

aquifer systems.

Figure 18. Distribution of volatile organic compounds (VOCs) detected in water samples by major land-use category within 1,000 feet 

of the well or spring, Virginia.

0246 8 10

Developed

Forested

Agricultural

NUMBER OF VOC PEAKS DETECTED

36

74

MAJOR LAND-USE CATEGORY

57

EXPLANATION 

Circles indicate outliers when number of samples

exceeds 10 and individual samples when number

of samples does not exceed 10.

10 90 25 50 75 

PERCENTILE

36 (Number of samples)
34 Aquifer Susceptibility in Virginia, 1998-2000

Figure 19. Susceptibility to contamination from near-surface sources of the regional aquifer systems in Virginia, 1998-2000.

39 ̊40 ̊38 ̊37 ̊

83 ̊ 82 ̊ 81 ̊ 80 ̊ 79 ̊ 78 ̊ 77 ̊ 

Fall

Line

County boundaries from 1:100,000, TIGER files, 1994

Base from USGS Digital Line Graph, 1:2,000,000, 1987

Universal Transverse Mercator 18 projection, NAD 27, central meridian 75o00'W

Province boundaries modified from Virginia Division of Mineral Resources, 2003

Regional aquifer systemAppalachian PlateausValley and Ridge CarbonateValley and Ridge SiliciclasticBlue RidgePiedmontCoastal Plain ShallowCoastal Plain MiddleCoastal Plain Deep

Susceptibility determinatioSusceptibleSusceptibleSusceptibleSusceptibleSusceptibleSusceptible (upper 100 feetNot SusceptibleNot Susceptible

NORTH CAROLINA TENNESSEE

WEST

 VIRGINIA

KENTUCKY

WASHINGT

MARYLAND

Exceedance, in percent

EXPLANATION

SUSCEPTIBLE

SUSCEPTIBILITY DETERMINATION

NOT SUSCEPTIBLE (below 100 feet)

Range

CFC

85

100

96

100100

41

00

3

H

100

9596

100

9332

50

Threshold 

concentration

Detection 

limit

NO3

1590406557321018

Samples

with CFC 

contamination,

in percent

156236595712

00

Samples

with VOCs 

detected,

in percent

626788947971

552

912

Young

4.9 - 40.53.6 - 39.52.7 - 48.5

0.8 - 24.55.0 - 28.5

1.9 - 45.4

ndnd

3

16

6

14

0.9

.8

ndnd

------

ndnd

100

969691

100

93

ndnd

Old

ndndndndnd

 2,420 - 10,100 9,090 - 36,90027,900 - 39,700

Apparent age1 Percentage young 

water fraction

1 Apparent age of young waters in years. Apparent age of old waters in radiocarbon years. 2 May be the result of analytical or sampling error, transient leakage from the well casing or wellbore, or remnants of well-construction or pump maintenance activities.

0 25 

0 25 5

VOCs

nd

ChlorofluorocarbonTritiumNitrateVolatile organic compoundsnot determined

CFC

3

H

NO3

Appalachian Plateaus

Valley and Ridge Carbonate and Siliciclastic

Blue Ridge

Piedmont

Coastal Plain Shallow, Middle, and Deep

VIRGINIA
Aquifer Susceptibility 35

The multiple tracer approach used during the VAS 

study provided a consistent means to identify the presence of young waters in the regional aquifer systems 

across Virginia. The low-level detections of the environmental tracers not only identified the presence of 

young waters, but also allowed for the determination of 

small fractions of young waters in binary mixtures with 

older (pre-CFC) waters. The ratio of the percentage and 

apparent age of the young fraction in binary mixtures 

provides an indication of the relative degree of susceptibility. The large ratios (greater than 10) depicted on 

figure 21 are associated with samples in which the fraction of the young water is large (greater than 75 percent) and apparent age is extremely young (less than 5 

years). Samples from the fractured-rock regional aquifer systems generally have the largest ratios. The abundance of large ratios in the Blue Ridge regional aquifer 

system is consistent with the findings from Plummer 

and others (2001), in which apparent ages for springs 

and wells in Shenandoah National Park ranged from 0 

to 3 and 0 to 25 years, respectively, and the percentage 

of young water fraction was generally greater than 50 

percent. Analysis-of-variance tests indicate that the 

ratios in the samples from the Blue Ridge regional 

aquifer system are significantly larger (p<0.05) than 

those in the other regional aquifer systems, whereas the 

ratios of the other regional aquifer systems could not be 

statistically separated.

Figure 20. Depth to top of first water zone and well depth in relation to exceedance and nonexceedance of susceptibility threshold 

concentrations by regional aquifer system, Virginia.

WELL DEPTH, IN FEET BELOW LAND SURFACE

EXPLANATION 

TOP OF FIRST WATER ZONE, IN

FEET BELOW LAND SURFACE

Appalachian Plateaus

SUSCEPTIBILITY THRESHOLD

CONCENTRATIONS EXCEEDED

Valley and Ridge Carbonate

Valley and Ridge Siliciclastic

Blue Ridge

Piedmont

Coastal Plain Shallow

Coastal Plain Middle

SUSCEPTIBILITY THRESHOLD

CONCENTRATIONS NOT EXCEEDED

Coastal Plain Shallow

Coastal Plain Middle

Coastal Plain Deep

0 200 400 600 800 1,000 1,200

0

200

400

600

800

1,000

1,200
36 Aquifer Susceptibility in Virginia, 1998-2000

SUMMARY AND CONCLUSIONS

The U.S. Geological Survey (USGS), in cooperation with the Virginia Department of Health, sampled 

water from 171 wells and springs across the Commonwealth of Virginia between 1998 and 2000 as part of 

the Virginia Aquifer Susceptibility (VAS) study. Most 

of the sites sampled are public water supplies that are 

part of the comprehensive Source Water Assessment 

Program for the Commonwealth. This report presents 

determinations of aquifer susceptibility to contamination from near-surface sources for the regional aquifers 

that serve as public water supplies and compares aquifer susceptibility determinations to apparent 

ground-water ages and to the occurrence of selected 

chemical constituents in ground water. The Commonwealth was subdivided into eight regional aquifer systems on the basis of (1) physiographic province, (2) 

geologic province, (3) hydrogeologic characteristics, 

(4) major rock type (in the Valley and Ridge Province), 

and (5) depth of the top of the first screened interval (in 

the Coastal Plain Province). These regional aquifer systems are:

1. Appalachian Plateaus

2. Valley and Ridge Carbonate

3. Valley and Ridge Siliciclastic

4. Blue Ridge

5. Piedmont (including the Mesozoic Basins)

6. Coastal Plain Shallow (depths less than 200 ft 

below land surface)

7. Coastal Plain Middle (depths between 200 and 

400 ft below land surface)

8. Coastal Plain Deep (depths greater than 400 ft 

below land surface).

The fundamental premise of the study was that the 

identification of young waters (less than 50 years) by 

multiple environmental tracers could be used as a guide 

for classifying aquifers in terms of susceptibility to 

contamination from near-surface sources. Multiple 

environmental tracers–chlorofluorocarbons (CFCs), 

sulfur hexafluoride (SF6), tritium (3H), and tritium/helium-3 (3H/3He)–and carbon isotopes (14C and 

δ13C) were used to determine the age of water discharging from wells and springs. Concentrations of 

CFCs greater than 5 pg/kg and 3H concentrations 

greater than 0.6 TU were used as thresholds to indicate 

that parts of the aquifer sampled have a component of 

Figure 21. Distribution of the ratio of the percentage and apparent age of the young water fraction in binary mixtures with old (pre-chlorofluorocarbon) 

waters by regional aquifer system, Virginia.

RATIO OF PERCENTAGE AND APPARENT AGE OF YOUNG WATER FRACTION

IN BINARY MIXTURES

REGIONAL AQUIFER SYSTEM

Appalachian

Plateaus

Valley and Ridge

Carbonate

Valley and Ridge

Siliciclastic

Blue Ridge

Piedmont

Coastal Plain

Shallow

10

17

22

12

21

16

0.01 0.1 1 10 100

EXPLANATION 

Circles indicate outliers when number of samples

exceeds 10 and individual samples when number

of samples does not exceed 10.

10 90 25 50 75 

PERCENTILE

10 (Number of samples)
Summary and Conclusions 37

young water and are, therefore, susceptible to near-surface contamination.

Concentrations of CFCs exceeded the susceptibility 

threshold in 22 percent of the wells and in one spring 

sampled in the Coastal Plain regional aquifer systems. 

About 74 percent of the samples from wells with the 

top of the first water zone less than 100 ft below land 

surface exceeded the threshold values, and water supplies developed in the upper 100 ft of the Coastal Plain 

are considered to be susceptible to contamination from 

near surface sources. The maximum depth to the top of 

the screened interval for wells that contained CFCs was 

less than 150 ft. The probability of water supplies 

being affected by near-surface sources of contamination is greatest in the upper 100 ft of the Coastal Plain 

Shallow regional aquifer system. The analyses of the 

sample from one spring and results from previous studies of springs in the Coastal Plain of Virginia indicate 

that springs in this shallow regional aquifer system 

contain young water, and, therefore, are susceptible to 

contamination from near-surface sources.

Wells completed in the deep confined aquifers in 

the Coastal Plain generally contain water older than 

1,000 years, as indicated by radiocarbon dating, and 

ranged from 9,090 to 39,700 radiocarbon years BP 

(before present where present is AD 1950). These 

regional aquifer systems are not considered to be susceptible to contamination under natural conditions. The 

low concentrations of CFCs and 3H detected in some of 

samples from the Coastal Plain Middle and Deep 

regional aquifer systems may be the result of analytical 

or sampling error, or transient leakage from the well 

casing or wellbore. The presence of paleowaters in the 

Coastal Plain regional aquifer systems is consistent 

with conceptual and flow-simulation models of this 

region, where ground-water recharge occurs along the 

Fall Line, and water flows towards the Atlantic Ocean 

along deep regional flow paths. The relation between 

increasing adjusted radiocarbon ages and distance from 

the Fall Line for samples from the Cretaceous aquifers 

(upper, middle, and lower Potomac aquifers) appears to 

differ with regard to a site’s orientation to the James 

River. Samples from north of the James River become 

older over a shorter distance from the Fall Line than 

those samples from south of the James River, which 

may be the result of possible bifurcation of flow around 

what is now known to be the Chesapeake Bay impact 

crater.

All of the water samples from wells and springs in 

the fractured-rock terrains (the Appalachian Plateaus, 

Valley and Ridge, Blue Ridge, and Piedmont regional 

aquifer systems) contained concentrations of CFCs and 

3H greater than one or both of the thresholds. Because 

all of the water samples exceeded at least one of the 

threshold values, young water is present throughout 

most of these regional aquifer systems; therefore, water 

supplies developed in these systems are susceptible to 

contamination from near-surface sources. No relation 

between well depth and presence of CFCs is evident 

from samples in the fractured-rock terrains.

More than 95 percent of the samples for which the 

dating methods were applicable contained waters with 

apparent ages less than 35 years. About 5 percent of 

these samples, most of which were from the Blue 

Ridge and Piedmont regional aquifer systems, contained young waters with apparent ages of less than 5 

years. Most of the samples from the Valley and Ridge 

Carbonate, Blue Ridge, and Piedmont regional aquifer 

systems had young water fractions of more than 50 percent, whereas samples from the Coastal Plain Shallow 

and Appalachian Plateaus regional aquifer systems 

contained less than 40 percent young waters. The 

apparent ages determined by the VAS study probably 

represent maximum ages of the young fractions 

because processes such as dilution and matrix diffusion 

can alter the original tracer concentrations as the 

ground water moves through the flow system. The 

actual ages of the young fractions, therefore, may be 

younger than the apparent ages presented in this report 

and the regional aquifer systems may be more susceptible, especially in the fractured-rock terrains, than is 

indicated by the findings of this study.

Concentrations of CFCs in excess of air-water equilibrium, which can indicate that nonatmospheric 

sources (such as sewage effluent) have introduced 

CFCs into the ground-water system, were measured in 

6 and 48 percent of the water samples from the Coastal 

Plain and fractured-rock regional aquifer systems, 

respectively. The presence of CFC contamination is 

another means of assessing the validity of the VAS 

methodology for determining aquifer susceptibility to 

near-surface contamination. None of the samples from 

the Coastal Plain Middle and Deep regional aquifer 

systems contained CFC contamination, and CFCs were 

not detected in a majority of the samples. The high percentage of samples with measured CFC contamination 

in the fractured-rock terrains indicates that supplies 
38 Aquifer Susceptibility in Virginia, 1998-2000

developed in these regional aquifer systems are susceptible. The absence of either CFCs or CFC contamination in the samples from the Coastal Plain Middle and 

Deep regional aquifer systems illustrates the lack of 

connection and the long period of time it takes water to 

flow to these aquifers from near land surface under natural conditions.

The nitrate (NO3) concentrations greater than the 

USGS detection level of 0.05 mg/L generally increase 

as the apparent age of the young water fraction 

decreases, with the highest NO3 concentrations for 

samples in which one or more of the CFCs are above 

modern atmospheric mixing ratios (commonly referred 

to as “contaminated” for ground-water dating purposes). Most of the samples in which NO3 was detected 

were from the fractured-rock regional aquifer systems, 

especially the Valley and Ridge Carbonate regional 

aquifer system, where 90 percent of the samples had 

concentrations greater than the detection level. Nitrate 

concentrations were equal to or slightly greater than the 

USGS detection levels for samples that contained only 

old or paleowaters. Although the evaluation of NO3

concentrations and the multiple tracer approach yielded 

somewhat consistent results in terms of aquifer susceptibility, the viability of NO3 concentrations as an indicator of aquifer susceptibility in the Appalachian 

Plateaus, Valley and Ridge Siliciclastic, and Coastal 

Plain Shallow regional aquifer systems can be limited 

by processes such as dilution and methanogenesis and 

by proximity of sampling sites to near-surface sources.

Numerous halogenated volatile organic compounds were detected at low concentrations (parts per 

quadrillion) in the samples from the regional aquifer 

systems in the fractured-rock terrains and the Coastal 

Plain Shallow regional aquifer system. The presence of 

VOCs in these samples is to be expected because the 

susceptibility threshold concentrations were exceeded 

and the regional aquifer systems are classified as susceptible to contamination from near-surface activities. 

The identification of the VOCs is only qualitative 

because quantitative analysis to determine actual concentrations is not possible with gas chromatography 

with an electron capture detector (GC-ECD). However, 

low-level detections of VOCs by GC-ECD can be used 

to indicate aquifer susceptibility and provide early 

warning of potential contamination of ground-water 

sources. The identification of VOC peaks in the samples from the Coastal Plain Middle and Deep regional 

aquifer systems is problematic and suggests that transient leakage from the well casing or wellbore is allowing young water to enter the well or that the VOCs are 

remnants from well-construction or pump-maintenance 

activities. Comparison of the number of VOCs detected 

and the major land-use category within 1,000 ft of the 

well or spring suggests that the occurrence of VOCs is 

only slightly greater in the developed areas than in forested and agricultural areas. The area contributing 

recharge to a well does not have to include the actual 

location of the well or the area encompassing a specified distance from the well.

The relative degree of susceptibility can be indicated by the ratio of the percentage and apparent age of 

the young fraction in binary mixtures of young and old 

waters. Large ratios are associated with samples in 

which the fraction of the young water is large (greater 

than 75 percent) and apparent age is extremely young 

(less than 5 years). Samples from the fractured-rock 

regional aquifer systems generally have the largest 

ratios. Analysis-of-variance tests indicate that the ratios 

in the samples from the Blue Ridge regional aquifer 

system are significantly higher (p<0.05) than those in 

the other regional aquifer systems.

Results from this study indicate that shallow wells 

(less than 100 ft deep) and springs in the Coastal Plain 

Shallow regional aquifer system and wells and springs 

in the fractured-rock terrains contain a component of 

young ground water and are, therefore, susceptible to 

contamination from near-surface sources.
References Cited 39

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Tables 45

Tables 5-6
46 Aquifer Susceptibility in Virginia, 1998-2000 Table 5. Apparent ages from chlorofluorocarbons, tritium/helium-3, and sulfur hexafluoride dating methods, and adjusted radiocarbon ages for water samples from wells 

[VAS, Virginia Aquifer Susceptibility study; Susc. Cat., Susceptibility category where "Y" indicates water sample exceeded CFC and tritium threshold values of 5 picogramtritium units (TU), respectively and "N" indicates water sample did not exceed threshold values. CFC-11, (trichlorofluoromethane, CFC13); CFC-12, (dichlorodifluorometha

(trichlorotrifluoroethane, C2F3Cl3); 3H/3He, tritium/helium-3; SF6, sulfur hexafluoride; 14C Adj. age, adjusted radiocarbon age in radiocarbon years BP (present is AD 1950)

are based on the Libby half-life of 5,568 years and have not been calibrated to calendar years; NP, not possible; >, older than age denoted; nd, not determined. See figure 1 fo

VAS no. Project1 Site type Regional aquifer system2 Susc. cat. Date Apparent age and uncertainty3, in years 14C Adj. age4 (years) 

CFC-11 CFC-12 CFC-113 CFC-11/ CFC-12 CFC-113/ CFC-12 CFC-113/ CFC-11 3H/3He SF6 Young 

AP-01 VAS Well AP Y 7/10/2000 C C C nd nd nd nd 36.0 ±1.8 nd nd AP-02 VAS Well AP Y 7/10/2000 40.5 ±1.8 45.5 ±1.8 >45.5 ±0.0 nd nd nd 4.9 ±0.8 36.0 ±1.5 nd 4.9 AP-03 VAS Well AP Y 7/20/2000 22.6 ±1.3 12.6 ±1.5 14.1 ±.5 NP NP nd nd C nd 14.1 AP-04 VAS Well AP Y 7/11/2000 48.0 ±1.8 49.0 ±3.3 >45.5 ±.0 47 nd nd nd 40.5 ±2.5 nd 40.5 AP-05 VAS Well AP Y 7/12/2000 45.5 ±.5 38.5 ±1.5 34.0 ±1.8 NP 20.5 nd nd 18.0 ±1.0 nd 20.5 AP-06 VAS Well AP Y 7/13/2000 44.0 ±.5 35.0 ±.3 32.0 ±1.8 NP 22.5 nd nd 16.0 ±.8 nd 22.5 AP-07 VAS Well AP Y 7/13/2000 47.5 ±1.0 >60.5 ± .0 >45.5 ±.0 nd nd nd 24.1 ± .3 36.0 ± .5 nd 24.1 AP-08 VAS Well AP Y 7/10/2000 26.0 ± .5 19.0 ±1.8 19.5 ± .8 NP NP 12.5 nd 18.0 ± .8 nd 19 AP-09 VAS Well AP Y 7/10/2000 37.5 ±1.0 43.0 ± .8 >45.5 ± .0 nd nd nd nd 18.5 ±1.3 nd 18.5 AP-10 VAS Well AP Y 7/11/2000 51.0 ±1.5 50.5 ±1.3 >45.5 ± .0 NP nd nd nd 24.5 ±1.0 nd 24.5 AP-11 VAS Well AP Y 7/11/2000 48.0 ± .8 47.0 ±1.0 >45.5 ± .0 NP nd nd nd 30.0 ±1.3 nd nd AP-12 VAS Well AP Y 7/12/2000 40.5 ± .5 C 28.0 ± .8 nd nd nd nd 12.5 ±1.3 nd 12.5 AP-13 VAS Well AP Y 7/12/2000 46.0 ± .3 51.0 ±1.8 >45.5 ± .0 30.5 nd nd nd 41.0 ±1.8 nd 30.5 BR-01 VAS Well BR Y 7/19/1999 30.0 ±1.8 23.5 ± .5 26.0 ± .5 NP nd 17 nd C nd 17 BR-02 VAS Well BR Y 7/19/1999 13.5 ±1.3 C 10.5 ± .8 nd nd 2.5 nd C nd 2.5 BR-03 VAS Well BR Y 7/20/1999 C C 9.1 ±1.0 nd nd nd 1.5 ± .2 5.1 ±1.3 nd 1.5 BR-04 VAS Well BR Y 7/20/1999 45.1 ±1.0 21.6 ± .8 >44.6 ± .0 NP nd nd nd 13.1 ± .8 nd 13 BR-05 VAS Well BR Y 8/25/1999 C C C nd nd nd 20.2 ±2.1 20.2 ±2.5 nd 20.2 BR-06 VAS Well BR Y 9/13/1999 32.7 ±1.3 C C nd nd nd 5.1 ± .2 C nd 5.1 BR-07 VAS Well BR Y 9/14/1999 29.7 ± .5 C C nd nd nd nd C nd nd BR-08 VAS Well BR Y 9/16/1999 C C C nd nd nd 2.2 ± .1 2.2 ±1.8 nd 2.2 BR-09 VAS Well BR Y 10/18/1999 40.8 ±1.0 19.3 ±1.0 12.8 ± .5 NP nd nd nd 9.3 nd 9.3 BR-10 VAS Well BR Y 10/26/1999 C C C nd nd nd nd 0.8 nd 0.8 CP-01 VAS Well CP-D N 6/23/1998 >53.5 ± .0 >58.5 ± .0 >43.5 ± .0 nd nd nd nd 18.0 ±1.0 31,800 ±2,300 nd CP-02 VAS Well CP-M N 6/24/1998 >53.5 ± .0 >58.5 ± .0 >43.5 ± .0 nd nd nd nd 27.5 ±1.0 28,700 ±1,900 nd CP-03 VAS Well CP-D N 6/25/1998 >53.5 ± .0 >58.5 ± .0 >43.5 ± .0 nd nd nd nd 34.5 ± .8 27,900 ±1,900 nd CP-04 VAS Well CP-M N 7/1/1998 >53.5 ± .0 >58.5 ± .0 >43.5 ± .0 nd nd nd nd 31.5 ± .5 34,600 ±1,900 nd 
Tables 47

CP-05 VAS Well CP-M N 7/6/1998 >53.5 ± .0 >58.5 ± .0 >43.5 ± .0 nd nd nd nd 26.0 ± .5 36,900 ±1,900 nd CP-06 VAS Well CP-D N 7/6/1998 >53.5 ± .0 >58.5 ± .0 >43.5 ± .0 nd nd nd nd 26.5 ± .8 33,400 ±1,900 nd CP-07 VAS Well CP-M N 7/7/1998 >53.5 ± .0 >58.5 ± .0 >43.5 ± .0 nd nd nd nd 35.0 ± .5 29,000 ±2,600 nd CP-08 VAS Well CP-M N 7/7/1998 >53.5 ± .0 >58.5 ± .0 >43.5 ± .0 nd nd nd nd 28.5 ±1.0 34,700 ±1,900 nd CP-09 VAS Well CP-S N 7/8/1998 >53.5 ± .0 >58.5 ± .0 >43.5 ± .0 nd nd nd nd 30.0 ± .8 nd nd CP-10 VAS Well CP-M N 7/8/1998 >53.5 ± .0 >58.5 ± .0 >43.5 ± .0 nd nd nd nd 30.0 ± .8 31,100 ±1,900 nd CP-11 VAS Well CP-S N 7/9/1998 >53.5 ± .0 >58.5 ± .0 >43.5 ± .0 nd nd nd nd 33.0 ± .5 10,100 ±2,800 nd CP-12 VAS Well CP-S N 7/14/1998 46.5 ±2.0 55.5 ±3.3 >43.5 ± .0 nd nd nd nd 37.5 ±1.0 nd 37.5 CP-13 VAS Well CP-S N 7/15/1998 >53.5 ± .0 >58.5 ± .0 >43.5 ± .0 nd nd nd nd >46.5 ± .0 nd nd CP-14 VAS Well CP-S Y 7/15/1998 11.0 ±1.0 C C nd nd nd 20.6 ± .4 15.5 ±1.0 nd 20.6 CP-15 VAS Well CP-S N 7/15/1998 >53.5 ± .0 >58.5 ± .0 >43.5 ± .0 nd nd nd nd 26.5 ± .8 nd nd CP-16 VAS Well CP-S N 7/16/1998 >53.5 ± .0 >58.5 ± .0 >43.5 ± .0 nd nd nd nd 35.5 ± .8 nd 35.5 CP-17 VAS Well CP-S N 7/16/1998 >53.5 ± .0 >58.5 ± .0 >43.5 ± .0 nd nd nd nd 21.0 ± .8 nd nd CP-18 VAS Well CP-M N 7/27/1998 47.6 ±1.3 >58.6 ± .0 >43.6 ± .0 nd nd nd nd 24.6 ± .5 10,100 ±2,800 nd CP-19 VAS Well CP-M N 7/27/1998 50.6 ± .5 >58.6 ± .0 >43.6 ± .0 nd nd nd nd 26.6 ± .8 9,790 ±2,500 26.6 CP-20 VAS Well CP-D N 7/28/1998 >53.6 ±0.0 >58.6 ±0.0 >43.6 ±0.0 nd nd nd nd 31.1 ±0.8 36,600 ±1,900 nd CP-21 VAS Well CP-S Y 7/29/1998 >53.6 ± .0 34.1 ± .3 >43.6 ± .0 NP nd nd 34.5 ±1.0 26.1 ± .8 2,420 ±2,800 26.1 CP-22 VAS Well CP-M N 7/30/1998 >53.6 ± .0 >58.6 ± .0 >43.6 ± .0 nd nd nd nd >46.6 ±2.3 9,090±3,000 nd CP-23 VAS Well CP-M N 8/3/1998 >53.6 ± .0 >58.6 ± .0 >43.6 ± .0 nd nd nd nd >46.6 ± .0 20,200 ±2,600 nd CP-24 VAS Well CP-D N 8/4/1998 >53.6 ± .0 >58.6 ± .0 >43.6 ± .0 nd nd nd nd 29.1 ± .8 34,200 ±1,900 29.1 CP-25 VAS Well CP-M N 8/4/1998 46.6 ±1.0 52.1 ± .8 43.6 ±1.3 21.6 12.6 12.6 nd 25.6 ± .8 23,600 ±1,900 12.6 CP-26 VAS Well CP-S Y 8/5/1998 C C C nd nd nd 12.0 ± .2 10.1 ±1.3 nd 12 CP-27 VAS Well CP-D N 8/6/1998 >53.6 ± .0 >58.6 ± .0 >43.6 ± .0 nd nd nd nd 30.6 ± .8 38,100 ±1,900 nd CP-28 VAS Well CP-S Y 8/17/1998 >53.6 ± .0 >58.6 ± .0 >43.6 ± .0 nd nd nd nd 21.1 ±1.0 nd 21.1 CP-29 VAS Well CP-M N 8/17/1998 >53.6 ± .0 >58.6 ± .0 >43.6 ± .0 nd nd nd nd 34.1 ± .5 16,700 ±3,100 nd CP-30 VAS Well CP-M N 8/18/1998 >53.6 ± .0 >58.6 ± .0 >43.6 ± .0 nd nd nd nd >46.6 ± .0 30,000 ±2,300 nd CP-31 VAS Well CP-M N 8/19/1998 >53.6 ± .0 >58.6 ± .0 >43.6 ± .0 nd nd nd nd 34.1 ± .5 10,100 ±2,900 nd 

VAS

no. Project1 Site type Regional aquifer system2 Susc. cat. Date Apparent age and uncertainty3, in years 14C Adj. age4 (years) 

CFC-11 CFC-12 CFC-113 CFC-11/ CFC-12 CFC-113/ CFC-12 CFC-113/ CFC-11 3H/3He SF6 Young 

Table 5. Apparent ages from chlorofluorocarbons, tritium/helium-3, and sulfur hexafluoride dating methods, and adjusted radiocarbon ages for water samples from wells 

1998-2000—Continued

[VAS, Virginia Aquifer Susceptibility study; Susc. Cat., Susceptibility category where "Y" indicates water sample exceeded CFC and tritium threshold values of 5 picogram

tritium units (TU), respectively and "N" indicates water sample did not exceed threshold values. CFC-11, (trichlorofluoromethane, CFC13); CFC-12, (dichlorodifluorometha

(trichlorotrifluoroethane, C2F3Cl3); 3H/3He, tritium/helium-3; SF6, sulfur hexafluoride; 14C Adj. age, adjusted radiocarbon age in radiocarbon years BP (present is AD 1950

are based on the Libby half-life of 5,568 years and have not been calibrated to calendar years; NP, not possible; >, older than age denoted; nd, not determined. See figure 1 f
48 Aquifer Susceptibility in Virginia, 1998-2000 CP-32 VAS Well CP-M Y 8/31/1998 >53.7 ± .0 >58.7 ± .0 >43.7 ± .0 nd nd nd nd >46.7 ± .0 24,800 ±2,900 nd 

CP-33 VAS Well CP-S N 8/31/1998 >53.7 ± .0 >58.7 ± .0 >43.7 ± .0 nd nd nd nd 38.2 ±1.3 nd nd CP-34 VAS Well CP-S Y 8/31/1998 42.7 ± .5 37.7 ± .8 26.7 ±3.3 NP nd nd nd 25.2 ±3.3 2,430±2,800 25.2 CP-35 VAS Well CP-D N 9/1/1998 >53.7 ± .0 >58.7 ± .0 >43.7 ± .0 nd nd nd nd 33.2 ± .5 33,900 ±2,800 nd CP-36 VAS Well CP-M N 9/1/1998 48.7 ± .5 52.2 ±2.4 >43.7 ± .0 43.2 nd nd nd 34.7 ±1.8 29,100 ±1,900 nd CP-37 VAS Well CP-M N 9/2/1998 >53.7 ± .0 >58.7 ± .0 >43.7 ± .0 nd nd nd nd 33.2 ± .8 21,100 ±3,100 nd CP-38 VAS Well CP-D N 9/2/1998 >53.7 ± .0 >58.7 ± .0 >43.7 ± .0 nd nd nd nd 30.7 ± .5 nd nd CP-39 VAS Well CP-M N 9/2/1998 >53.7 ± .0 >58.7 ± .0 >43.7 ± .0 nd nd nd nd 38.7 ±1.8 11,900 ±3,100 nd CP-40 VAS Well CP-D N 9/3/1998 47.7 ± .8 52.2 ± .3 >43.7 ± .0 41.2 nd nd nd 40.2 ±2.3 28,000 ±2,200 nd CP-41 VAS Well CP-M N 9/10/1998 >53.7 ± .0 >58.7 ± .0 >43.7 ± .0 nd nd nd nd 31.7 ± .5 36,900 ±1,900 nd CP-42 VAS Well CP-D N 9/10/1998 >53.7 ± .0 >58.7 ± .0 >43.7 ± .0 nd nd nd nd 34.2 ±1.3 30,800 ±1,900 nd CP-43 VAS Well CP-S N 10/1/1998 >53.8 ± .0 >58.8 ± .0 >43.8 ± .0 nd nd nd nd 36.8 ±1.0 nd nd CP-44 VAS Well CP-S N 10/5/1998 50.8 ±2.1 >58.8 ± .0 >43.8 ± .0 nd nd nd nd 30.8 ± .5 nd nd CP-45 VAS Well CP-S Y 10/5/1998 >53.8 ± .0 22.8 ± .8 >43.8 ± .0 NP nd nd 20.3 ± .5 9.3 ± .8 nd 9.3 CP-46 VAS Well CP-S Y 10/6/1998 22.8 ± .8 20.8 ±1.8 14.8 ± .8 NP 11.8 8.8 23.2 ± .4 19.3 ± .8 nd 11.8 CP-47 VAS Well CP-M N 10/6/1998 51.8 ±1.8 53.8 ±3.6 >43.8 ± .0 48.8 nd nd nd 35.8 ± .8 13,500 ±2,500 nd CP-48 VAS Well CP-S Y 10/7/1998 C C C nd nd nd nd C nd nd CP-49 VAS Well CP-S N 10/27/1998 50.8 ± .5 >58.8 ± .0 >43.8 ± .0 nd nd nd nd 36.3 ±2.0 nd 36 CP-50 VAS Spring CP-S Y 10/28/1998 9.8 ±2.9 C C nd nd nd 1.9 ± .2 4.3 ±1.5 nd 1.9 CP-51 VAS Well CP-D N 11/4/1998 >53.8 ± .0 >58.8 ± .0 >43.8 ± .0 nd nd nd nd 34.3 ± .8 39,700 ±1,900 nd PD-01 VAS Well PD Y 6/28/1999 C 19.5 ±1.8 18.0 ± .8 nd nd nd 42.6 ±9.2 C nd 18 PD-02 VAS Well PD Y 6/29/1999 45.5 ±1.0 26.5 ± .8 34.5 ±1.3 NP nd nd 40.6 ± .8 7.5 ±1.3 nd 7.5 PD-03 VAS Well PD Y 6/30/1999 36.5 ± .8 17.0 ±2.5 28.0 ± .3 NP nd nd 25.6 ± .4 17.5 ± .8 nd 25.6 PD-04 VAS Well PD Y 6/30/1999 35.5 ± .5 C 26.0 ± .5 nd nd nd 21.8 ± .5 C nd 21.8 PD-05 VAS Well PD Y 7/1/1999 C C 27.0 ±2.5 nd nd nd 15.9 ± .5 C nd 15.9 PD-06 VAS Well PD Y 8/23/1999 28.6 ± .5 C C nd nd nd 10.0 ± .3 14.1 ±1.0 nd 10 PD-07 VAS Well PD Y 8/23/1999 37.6 ± .5 16.6 ±1.0 C NP nd nd 31.5 ±1.3 18.6 nd 18.6 

VAS

no. Project1 Site type Regional aquifer system2 Susc. cat. Date Apparent age and uncertainty3, in years 14C Adj. age4 (years) 

CFC-11 CFC-12 CFC-113 CFC-11/ CFC-12 CFC-113/ CFC-12 CFC-113/ CFC-11 3H/3He SF6 Young 

Table 5. Apparent ages from chlorofluorocarbons, tritium/helium-3, and sulfur hexafluoride dating methods, and adjusted radiocarbon ages for water samples from wells 

1998-2000—Continued

[VAS, Virginia Aquifer Susceptibility study; Susc. Cat., Susceptibility category where "Y" indicates water sample exceeded CFC and tritium threshold values of 5 picogram

tritium units (TU), respectively and "N" indicates water sample did not exceed threshold values. CFC-11, (trichlorofluoromethane, CFC13); CFC-12, (dichlorodifluorometha

(trichlorotrifluoroethane, C2F3Cl3); 3H/3He, tritium/helium-3; SF6, sulfur hexafluoride; 14C Adj. age, adjusted radiocarbon age in radiocarbon years BP (present is AD 1950

are based on the Libby half-life of 5,568 years and have not been calibrated to calendar years; NP, not possible; >, older than age denoted; nd, not determined. See figure 1 f
Tables 49

PD-08 VAS Well PD Y 8/30/1999 14.7 ±1.0 11.2 ±1.3 C NP nd nd nd C nd nd PD-09 VAS Well PD Y 8/31/1999 40.7 ± .8 29.2 ± .3 12.2 ±1.0 NP nd nd nd 10.2 ±2.5 nd nd PD-10 VAS Spring PD Y 9/1/1999 C C C nd nd nd 11.0 ± .2 13.7 ±1.5 nd 11 PD-11 VAS Well PD Y 9/1/1999 C C C nd nd nd 16.2 ± .2 C nd 16.2 PD-12 VAS Well PD Y 9/1/1999 47.2 ± .5 24.7 ± .5 14.7 ±1.5 NP nd nd nd C nd 14.7 PD-13 VAS Spring PD Y 9/2/1999 15.2 ±1.3 8.2 ±2.8 C NP nd nd 17.2 ±0.2 13.7 ±0.8 nd 17.2 PD-14 VAS Well PD Y 9/2/1999 19.2 ±1.0 8.2 ±3.6 9.2 ±1.5 NP NP nd 14.6 ± .2 10.7 ± .8 nd 14.6 PD-15 VAS Well PD Y 9/2/1999 36.7 ± .3 23.7 ± .5 11.7 ± .8 NP nd nd 24.0 ± .2 C nd nd PD-16 VAS Well PD Y 9/13/1999 27.2 ± .5 C C nd nd nd nd C nd nd PD-17 VAS Well PD Y 9/13/1999 27.2 ±1.5 27.7 ± .5 C 24.2 nd nd nd 17.7 ±1.0 nd 17.7 PD-18 VAS Well PD Y 10/18/1999 39.3 ± .5 21.3 ±1.0 C NP nd nd nd 11.8 ±1.5 nd 11.8 PD-19 VAS Well PD Y 10/19/1999 25.3 ± .5 20.3 ±1.3 12.3 ± .5 NP nd nd 19.3 ± .3 14.3 ± .8 nd 14.3 PD-20 VAS Well PD Y 10/19/1999 42.3 ± .5 C 15.8 ±1.0 nd nd nd 17.0 ± .2 19.3 ±1.8 nd 19.3 PD-21 VAS Well PD Y 10/25/1999 47.8 ± .5 46.3 ± .3 18.8 ±1.8 NP nd nd nd 28.3 ±1.0 nd 28.3 PD-22 VAS Well PD Y 10/25/1999 21.8 ±1.0 22.8 ± .5 17.3 ± .5 nd 13.3 14.3 nd 14.8 ±1.0 nd 14.3 PD-23 VAS Well PD Y 10/25/1999 C C C nd nd nd nd C nd nd PD-24 VAS Well PD Y 10/26/1999 C C C nd nd nd nd C nd nd PD-25 VAS Well PD Y 6/27/2000 14.5 ±1.0 C 24.5 ± .5 nd nd nd 26.5 ± .2 21.5 ± .5 nd 21.5 PD-26 VAS Well PD Y 6/27/2000 28.5 ± .5 13.5 ±1.5 20.0 ± .5 NP NP 8.5 nd C nd 14.5 PD-27 VAS Well PD Y 6/28/2000 37.0 ± .3 25.5 ±1.8 25.5 ±3.5 NP NP nd nd C nd 25.5 PD-28 VAS Well PD Y 6/28/2000 21.0 ±2.3 27.5 ±1.0 28.5 ± .5 nd nd nd nd 1 nd 28.5 PD-29 VAS Well PD Y 6/29/2000 48.5 ±1.3 C >45.5 ± .0 nd nd nd 21.6 ± .3 22.0 ±4.6 nd 22 PD-30 VAS Well PD Y 6/29/2000 40.5 ± .3 32.5 ± .5 27.5 ± .5 NP 18.5 nd 21.5 ±1.3 22.0 ± .5 nd 21.5 VB-01 VBCH Well CP-S N 5/13/1999 48.4 ± .0 52.9 ± .3 >44.4 ± .0 35.9 nd nd nd >47.4 ± .0 nd 35.9 VB-02 VBCH Well CP-S Y 5/14/1999 48.4 ±2.3 57.9 ±2.3 >44.4 ± .0 nd nd nd nd 33.9 ±1.0 nd 33.9 VB-03 VBCH Well CP-S Y 5/10/1999 49.9 ±1.0 28.9 ±1.0 >44.4 ± .0 NP nd nd nd 33.4 ± .5 nd 28.9 VB-04 VBCH Well CP-S N 5/12/1999 50.4 ± .3 53.9 ±3.4 >44.4 ± .0 45.4 nd nd nd >47.4 ±2.0 nd 45.4 

VAS

no. Project1 Site type Regional aquifer system2 Susc. cat. Date Apparent age and uncertainty3, in years 14C Adj. age4 (years) 

CFC-11 CFC-12 CFC-113 CFC-11/ CFC-12 CFC-113/ CFC-12 CFC-113/ CFC-11 3H/3He SF6 Young 

Table 5. Apparent ages from chlorofluorocarbons, tritium/helium-3, and sulfur hexafluoride dating methods, and adjusted radiocarbon ages for water samples from wells 

1998-2000—Continued

[VAS, Virginia Aquifer Susceptibility study; Susc. Cat., Susceptibility category where "Y" indicates water sample exceeded CFC and tritium threshold values of 5 picogram

tritium units (TU), respectively and "N" indicates water sample did not exceed threshold values. CFC-11, (trichlorofluoromethane, CFC13); CFC-12, (dichlorodifluorometha

(trichlorotrifluoroethane, C2F3Cl3); 3H/3He, tritium/helium-3; SF6, sulfur hexafluoride; 14C Adj. age, adjusted radiocarbon age in radiocarbon years BP (present is AD 1950

are based on the Libby half-life of 5,568 years and have not been calibrated to calendar years; NP, not possible; >, older than age denoted; nd, not determined. See figure 1 f
50 Aquifer Susceptibility in Virginia, 1998-2000 VB-05 VBCH Well CP-S N 8/14/2000 53.6 ±2.0 >60.6 ± .0 >45.6 ± .0 nd nd nd nd 32.6 ±2.0 nd nd 

VB-06 VBCH Well CP-S N 8/11/2000 52.6 ± .5 52.6 ± .8 >45.6 ± .0 NP nd nd nd 29.1 ±1.5 nd 29.1 VB-07 VBCH Well CP-S Y 8/10/2000 35.6 ± .8 45.6 ±1.5 24.6 ±1.0 nd nd nd nd 22.6 ± .5 nd nd VB-08 VBCH Well CP-S Y 8/9/2000 >55.6 ± .0 49.1 ± .8 32.6 ±1.5 NP nd nd nd 33.1 ± .5 nd 33 VB-09 VBCH Well CP-S Y 8/16/2000 48.1 ±5.5 50.1 ±2.0 35.1 ±1.3 45.6 nd nd nd 33.6 ± .8 nd 33.5 VB-10 VBCH Well CP-S N 8/17/2000 51.6 ±2.8 52.1 ±2.0 >45.6 ± .0 51.1 nd nd nd 26.6 ± .5 nd 26.5 VB-11 VBCH Well CP-S Y 8/16/2000 42.1 ±6.7 49.1 ±3.0 34.1 ±5.6 nd nd nd nd 26.1 ±1.0 nd nd VB-12 VBCH Well CP-S Y 8/15/2000 52.1 ±1.0 46.1 ± .5 27.1 ± .5 NP nd nd nd 29.6 ±1.0 nd 29.5 VB-13 VBCH Well CP-S N 8/8/2000 48.6 ± .5 >60.6 ± .0 >45.6 ± .0 nd nd nd nd C nd nd VB-14 VBCH Well CP-S N 8/7/2000 >55.6 ± .0 >60.6 ± .0 >45.6 ± .0 nd nd nd nd C nd nd VR-01 VAS Well VR-C Y 7/6/1999 47.0 ±1.5 >59.5 ± .0 39.5 ±5.5 nd nd nd nd 13.5 ± .8 nd 39.5 VR-02 VAS Well VR-C Y 7/6/1999 26.0 ± .3 26.5 ±1.8 21.0 ±2.0 22.5 15 15.5 9.2 ± .3 -0.5 nd 9.2 VR-03 VAS Well VR-C Y 7/7/1999 14.0 ±1.0 8.0 ±3.3 C NP nd nd 19.1 ± .9 C nd 8 VR-04 VAS Spring VR-C Y 7/7/1999 19.5 ±1.0 20.0 ±1.3 C NP nd nd nd C nd 20 VR-05 VAS Well VR-C Y 7/8/1999 14.0 ±1.0 C C nd nd nd 6.0 ± .2 C nd 6 VR-06 VAS Well VR-C Y 7/8/1999 24.0 ± .8 11.0 ±1.5 C NP nd nd 11.7 ± .4 C nd 11.7 VR-07 VAS Well VR-C Y 7/8/1999 29.5 ± .3 22.5 ±1.0 C NP nd nd 18.2 ±1.1 2.0 ±1.0 nd 18.2 VR-08 VAS Well VR-C Y 7/9/1999 34.0 ± .5 27.0 ± .3 C NP nd nd 43.0 ±2.1 2.5 ±1.5 nd nd VR-09 VAS Well VR-C Y 7/9/1999 29.0 ± .3 25.5 ±1.3 21.0 ± .5 NP 16.5 9 29.7 ± .7 1.0 ±1.5 nd 16.5 VR-10 VAS Well VR-C Y 7/9/1999 23.0 ± .8 15.5 ±1.0 21.0 ± .5 NP NP 19.5 16.1 ±1.1 C nd 19.5 VR-11 VAS Well VR-S Y 7/9/1999 49.0 ± .3 50.0 ±3.5 24.5 ±1.3 48.5 nd nd nd 15.0 ± .8 nd 48.5 VR-12 VAS Well VR-C Y 7/21/1999 24.6 ± .3 C 23.6 ± .5 nd nd 22.1 nd C nd 22.1 VR-13 VAS Well VR-C Y 7/21/1999 C C C nd nd nd 7.2 ± .2 7.1 ±1.3 nd 7.2 VR-14 VAS Spring VR-C Y 7/22/1999 34.6 ± .5 34.6 ± .8 C NP nd nd 24.3 ± .5 1.1 ±1.0 nd 24.3 VR-15 VAS Well VR-S Y 8/10/1999 48.6 ± .8 50.6 ±1.3 C 46.6 nd nd nd 36.6 ±0.5 nd 46.6 VR-16 VAS Well VR-S Y 8/11/1999 45.6 ± .3 41.1 ± .3 C NP nd nd nd 22.6 ± .5 nd nd VR-17 VAS Well VR-C Y 8/11/1999 13.6 ±1.0 3.1 ±4.2 C NP nd nd 12.4 ±0.3 C nd 12.4 

VAS

no. Project1 Site type Regional aquifer system2 Susc. cat. Date Apparent age and uncertainty3, in years 14C Adj. age4 (years) 

CFC-11 CFC-12 CFC-113 CFC-11/ CFC-12 CFC-113/ CFC-12 CFC-113/ CFC-11 3H/3He SF6 Young 

Table 5. Apparent ages from chlorofluorocarbons, tritium/helium-3, and sulfur hexafluoride dating methods, and adjusted radiocarbon ages for water samples from wells 

1998-2000—Continued

[VAS, Virginia Aquifer Susceptibility study; Susc. Cat., Susceptibility category where "Y" indicates water sample exceeded CFC and tritium threshold values of 5 picogram

tritium units (TU), respectively and "N" indicates water sample did not exceed threshold values. CFC-11, (trichlorofluoromethane, CFC13); CFC-12, (dichlorodifluorometha

(trichlorotrifluoroethane, C2F3Cl3); 3H/3He, tritium/helium-3; SF6, sulfur hexafluoride; 14C Adj. age, adjusted radiocarbon age in radiocarbon years BP (present is AD 1950

are based on the Libby half-life of 5,568 years and have not been calibrated to calendar years; NP, not possible; >, older than age denoted; nd, not determined. See figure 1 f
Tables 51

VR-18 VAS Well VR-S Y 8/12/1999 42.1 ± .5 32.6 ± .5 C NP nd nd nd 41.1 nd nd VR-19 VAS Well BR Y 8/24/1999 22.1 ±1.3 C 9.6 ±1.3 nd nd nd 26.6 ± .3 19.2 ±1.0 nd 19 VR-20 VAS Well VR-C Y 8/24/1999 16.1 ±1.3 13.1 ±1.0 C NP nd nd 3.6 ± .2 4.7 ±1.5 nd 3.6 VR-21 VAS Well VR-C Y 8/25/1999 29.1 ± .5 24.1 ±1.0 C NP nd nd 15.4 ± .4 C nd 15.4 VR-22 VAS Well VR-S Y 8/26/1999 25.7 ± .5 24.2 ± .5 C NP nd nd 15.7 ± .3 9.2 ±1.3 nd 9 VR-23 VAS Well VR-S Y 8/26/1999 24.7 ± .3 C C nd nd nd nd C nd nd VR-24 VAS Well VR-S Y 10/27/1999 47.3 ±2.3 49.8 ± .5 28.3 ±1.8 44.3 nd nd nd 33.3 ±1.5 nd 33 VR-25 VAS Well VR-C Y 10/27/1999 44.3 ± .5 37.8 ± .5 23.3 ±1.3 NP nd nd nd 19.8 ± .5 nd 20 VR-26 VAS Spring VR-S Y 10/28/1999 20.3 ±1.0 17.3 ±1.0 14.3 ± .5 NP 12.3 10.3 15.0 ± .3 15.3 ± .8 nd 15 VR-27 VAS Well VR-S Y 10/28/1999 23.3 ± .5 18.8 ±1.0 15.3 ± .5 NP 12.3 4.8 nd 8.8 ± .8 nd 9 VR-28 VAS Well VR-S Y 7/12/2000 46.0 ± .3 41.0 ± .3 39.5 ±1.5 NP 26.5 nd 2.7 ± .3 23.0 ± .5 nd 2.7 VR-29 VAS Well VR-S Y 7/13/2000 46.5 ± .3 48.0 ± .8 >45.5 ± .0 45.5 nd nd nd 32.0 ± .8 nd 32 VR-30 VAS Well VR-S Y 7/13/2000 34.5 ± .3 31.5 ± .5 25.0 ± .8 NP 15.5 nd nd 23.0 ± .8 nd 15.5 VR-31 VAS Well VR-S Y 7/17/2000 45.0 ± .5 48.5 ±1.8 >45.5 ± .0 36.5 nd nd nd 32.5 ± .5 nd 32.5 VR-32 VAS Well VR-S Y 7/17/2000 42.0 ± .8 31.5 ± .5 35.5 ±2.5 NP nd nd 23.5 ±1.0 25.5 ± .5 nd 23.5 VR-33 VAS Well VR-S Y 7/18/2000 17.0 ±1.3 C 11.5 ±1.5 nd nd nd 3.4 ± .2 C nd 3.4 VR-34 VAS Well VR-S Y 7/18/2000 34.5 ± .5 25.0 ±1.0 17.5 ±1.0 NP 12 nd nd 7.6 ±1.3 nd 12 VR-35 VAS Well VR-S Y 7/19/2000 51.1 ± .8 52.1 ±2.5 >45.6 ± .0 50.1 nd nd 13.3 ± .5 36.1 ± .8 nd 36 VR-36 VAS Well VR-S Y 7/20/2000 16.1 ±1.0 11.6 ±2.0 12.6 ± .5 NP NP 9.6 3.3 ± .4 8.1 ±1.5 nd 3.3 VR-37 VAS Well VR-S Y 7/17/2000 30.0 ± .5 29.0 ± .5 23.0 ±1.3 NP 16 12.5 23.8 ± .3 10.0 ±1.0 nd 16 VR-38 VAS Well VR-C Y 7/18/2000 25.5 ± .5 19.5 ± .8 16.5 ± .8 NP 15 3.5 13.4 ± .3 C nd 13.4 VR-39 VAS Well VR-S Y 7/18/2000 31.5 ± .5 32.5 ± .8 26.5 ± .5 28.5 16.5 15.5 17.8 ± .3 23.6 ± .5 nd 17.8 VR-40 VAS Well VR-S Y 7/19/2000 C 36.6 ± .5 25.1 ± .5 nd nd nd 18.6 ± .3 26.1 ±1.0 nd 18.6 VR-41 VAS Well VR-C Y 7/19/2000 36.6 ± .3 38.1 ± .5 23.1 ± .5 30.1 nd nd nd 23.6 ± .5 nd 23.5 VR-42 VAS Well VR-S Y 7/20/2000 16.6 ±1.0 C 13.6 ± .5 nd nd 11.1 nd C nd 11.1 VR-43 VAS Well VR-S Y 7/20/2000 37.6 ± .5 32.6 ± .5 27.6 ±1.0 NP 18.1 nd 19.0 ± .3 16.6 ±1.8 nd 18 VR-44 VAS Well VR-C Y 7/20/2000 12.6 ±1.5 C 15.6 ± .5 nd nd NP 15.3 ± .2 C nd 15.3 

VAS

no. Project1 Site type Regional aquifer system2 Susc. cat. Date Apparent age and uncertainty3, in years 14C Adj. age4 (years) 

CFC-11 CFC-12 CFC-113 CFC-11/ CFC-12 CFC-113/ CFC-12 CFC-113/ CFC-11 3H/3He SF6 Young 

Table 5. Apparent ages from chlorofluorocarbons, tritium/helium-3, and sulfur hexafluoride dating methods, and adjusted radiocarbon ages for water samples from wells 

1998-2000—Continued

[VAS, Virginia Aquifer Susceptibility study; Susc. Cat., Susceptibility category where "Y" indicates water sample exceeded CFC and tritium threshold values of 5 picogram

tritium units (TU), respectively and "N" indicates water sample did not exceed threshold values. CFC-11, (trichlorofluoromethane, CFC13); CFC-12, (dichlorodifluorometha

(trichlorotrifluoroethane, C2F3Cl3); 3H/3He, tritium/helium-3; SF6, sulfur hexafluoride; 14C Adj. age, adjusted radiocarbon age in radiocarbon years BP (present is AD 1950

are based on the Libby half-life of 5,568 years and have not been calibrated to calendar years; NP, not possible; >, older than age denoted; nd, not determined. See figure 1 f
52 Aquifer Susceptibility in Virginia, 1998-2000 VR-45 VAS Well VR-C Y 7/25/2000 28.1 ± .5 28.6 ± .5 26.6 ± .3 26.6 23.1 22.6 29.9 ± .4 C nd 23.1 

VR-46 VAS Well VR-S Y 7/25/2000 29.6 ± .5 29.1 ± .5 21.1 ± .8 NP 13.1 9.6 nd 11.1 ±2.0 nd 13.1 VR-47 VAS Well VR-S Y 7/26/2000 13.6 ±1.0 10.1 ±2.0 C NP nd nd 12.1 ± .3 C nd 12.1 VTDW-01 VPI Well BR Y 9/16/1999 19.7 ±5.0 8.7 ±2.8 C NP nd nd 2.4 ± .2 4.7 ±1.8 nd 2.4 VTDW-03A VPI Well BR Y 7/15/2000 31.5 ±1.5 29.5 ±1.3 22.5 ±1.8 NP 14.5 nd 18.2 ± .3 22.0 ± .8 nd 22 VTDW-03B VPI Well BR Y 7/15/2000 30.5 ±1.0 28.0 ±1.0 21.5 ±1.0 NP 14.5 nd 20.4 ± .4 18.0 ±1.0 nd 18 VTDW-07A VPI Well BR Y 7/14/2000 39.5 ± .5 28.5 ± .5 27.5 ±1.5 NP 24.5 nd 26.4 ± .3 20.0 ±1.0 nd 24.5 VTDW-07B VPI Well BR Y 7/14/2000 33.0 ± .3 30.0 ± .5 24.0 ±1.0 NP 16 nd nd C nd nd VTDW-08 VPI Well BR Y 9/16/1999 C C C nd nd nd 2.7 ± .2 4.7 ±1.5 nd 2.7 

VAS

no. Project1 Site type Regional aquifer system2 Susc. cat. Date Apparent age and uncertainty3, in years 14C Adj. age4 (years) 

CFC-11 CFC-12 CFC-113 CFC-11/ CFC-12 CFC-113/ CFC-12 CFC-113/ CFC-11 3H/3He SF6 Young 

1 Project designation: VAS, Virginia Aquifer Susceptibility study; VBCH, Virginia Beach shallow ground water study (Johnson, 1999); VPI, Virginia Polytechnic and State University fractured-rock 2 VAS Aquifer System designation: AP, Appalachian Plateaus; BR, Blue Ridge; CP-D, Coastal Plain Deep system (depths greater than 400 feet); CP-M, Coastal Plain Middle system (depths betwee

Shallow system (depths less than 200 feet); PD, Piedmont; VR-C, Valley and Ridge Carbonate; VR-S, Valley and Ridge Siliciclastic

3 Apparent CFC age uncertainties are based on changes in age resulting from uncertainty in nitrogen-argon recharge temperature of ±1oC. Apparent SF6 age uncertainties are based on changes in age 

nitrogen-argon excess air concentrations of recharge temperature of ±1 cc/L. 

4 Radiocarbon age uncertainties are based on ±5‰ variation in the δ13C in soil gas carbon dioxide input for the Fontes and Garnier (1979) model.5 Young water is defined as less than 50 years and old water is defined as pre-CFC.

Table 5. Apparent ages from chlorofluorocarbons, tritium/helium-3, and sulfur hexafluoride dating methods, and adjusted radiocarbon ages for water samples from wells 

1998-2000—Continued

[VAS, Virginia Aquifer Susceptibility study; Susc. Cat., Susceptibility category where "Y" indicates water sample exceeded CFC and tritium threshold values of 5 picogram

tritium units (TU), respectively and "N" indicates water sample did not exceed threshold values. CFC-11, (trichlorofluoromethane, CFC13); CFC-12, (dichlorodifluorometha

(trichlorotrifluoroethane, C2F3Cl3); 3H/3He, tritium/helium-3; SF6, sulfur hexafluoride; 14C Adj. age, adjusted radiocarbon age in radiocarbon years BP (present is AD 1950

are based on the Libby half-life of 5,568 years and have not been calibrated to calendar years; NP, not possible; >, older than age denoted; nd, not determined. See figure 1 f
Tables 53

VAS

no. Project1 Site type Regional aquifer system Susc. cat. Date Percentage of young fraction 

CFC-11 CFC-12 CFC-113 SF6 CFC-11/ CFC-12 CFC-113/ CFC-12 CFC-113/ CFC-11 Average

AP-01 VAS Well AP Y 7/10/2000 nd nd nd nd nd nd nd nd AP-02 VAS Well AP Y 7/10/2000 3.9 3.3 0 3.2 nd nd nd 3.4 AP-03 VAS Well AP Y 7/20/2000 69.7 110.9 107.8 426.9 NP NP nd 69.7 AP-04 VAS Well AP Y 7/11/2000 16.4 30.3 0 102.6 76.8 nd nd 23.4 AP-05 VAS Well AP Y 7/12/2000 2.4 15.4 15.7 140.2 NP 15.7 nd 15.7 AP-06 VAS Well AP Y 7/13/2000 4 27.4 28 222.8 NP 27.8 nd 27.8 AP-07 VAS Well AP Y 7/13/2000 1.5 0 0 24 nd nd nd 24 AP-08 VAS Well AP Y 7/10/2000 60.9 101.2 99.5 118.5 NP NP 44.5 nd AP-09 VAS Well AP Y 7/10/2000 9.8 7.7 0 100.9 nd nd nd 7.7 AP-10 VAS Well AP Y 7/11/2000 0.4 3 0 102 NP nd nd 3 AP-11 VAS Well AP Y 7/11/2000 nd nd nd nd NP nd nd nd AP-12 VAS Well AP Y 7/12/2000 4.4 1,393.0 13.6 100.1 nd nd nd 13.6 AP-13 VAS Well AP Y 7/12/2000 5.4 5.3 0 27.9 5.3 nd nd 5.3 BR-01 VAS Well BR Y 7/19/1999 30.8 69.7 31.6 2,763.9 NP nd 31.5 31.5 BR-02 VAS Well BR Y 7/19/1999 84.8 113.1 85.8 nd nd nd 85.7 85.7 BR-03 VAS Well BR Y 7/20/1999 877.5 107.1 94.3 79.2 nd nd nd 86.7 BR-04 VAS Well BR Y 7/20/1999 1.5 67.8 0 100 NP nd nd 67.8 BR-05 VAS Well BR Y 8/25/1999 809.7 14,289.1 1,965.3 111.7 nd nd nd nd BR-06 VAS Well BR Y 9/13/1999 14.1 396 248.6 0 nd nd nd 14.1 BR-07 VAS Well BR Y 9/14/1999 nd nd nd nd nd nd nd nd BR-08 VAS Well BR Y 9/16/1999 1,636.5 35,447.6 275.7 101.6 nd nd nd nd BR-09 VAS Well BR Y 10/18/1999 3.4 64.8 76.2 104.9 NP nd nd 70.5 BR-10 VAS Well BR Y 10/26/1999 764.5 4,375.8 4,910.3 102.9 nd nd nd nd CP-01 VAS Well CP-D N 6/23/1998 nd nd nd nd nd nd nd nd CP-02 VAS Well CP-M N 6/24/1998 nd nd nd nd nd nd nd nd CP-03 VAS Well CP-D N 6/25/1998 nd nd nd nd nd nd nd nd CP-04 VAS Well CP-M N 7/1/1998 nd nd nd nd nd nd nd nd CP-05 VAS Well CP-M N 7/6/1998 nd nd nd nd nd nd nd nd CP-06 VAS Well CP-D N 7/6/1998 nd nd nd nd nd nd nd nd 

Table 6. Percentage of young fraction in water samples from wells and springs in Virginia, 1998-2000[VAS, Virginia Aquifer Susceptibility study; Susc. Cat., Susceptibility category where "Y" indicates water sample exceeded chlorofluorocarbon (CFC) and tritium threshold values of 5 picograms pe

(TU), respectively and "N" indicates water sample did not exceed threshold values. CFC-11, (trichlorofluoromethane, CFC13); CFC-12, (dichlorodifluoromethane, CF2Cl2); CFC-113, (trichlorotrif

hexafluoride; NP, not possible; nd, not determined. See figure 1 for location of wells and springs]
54 Aquifer Susceptibility in Virginia, 1998-2000 CP-07 VAS Well CP-M N 7/7/1998 nd nd nd nd nd nd nd nd 

CP-08 VAS Well CP-M N 7/7/1998 nd nd nd nd nd nd nd nd CP-09 VAS Well CP-S N 7/8/1998 nd nd nd nd nd nd nd nd CP-10 VAS Well CP-M N 7/8/1998 nd nd nd nd nd nd nd nd CP-11 VAS Well CP-S N 7/9/1998 nd nd nd nd nd nd nd nd CP-12 VAS Well CP-S N 7/14/1998 11.3 2.3 0 102.8 nd nd nd 2.3 CP-13 VAS Well CP-S N 7/15/1998 nd nd nd nd nd nd nd nd CP-14 VAS Well CP-S Y 7/15/1998 166.4 384 2,924.9 206.3 nd nd nd nd CP-15 VAS Well CP-S N 7/15/1998 nd nd nd nd nd nd nd nd CP-16 VAS Well CP-S N 7/16/1998 0 0 0 105.5 nd nd nd nd CP-17 VAS Well CP-S N 7/16/1998 nd nd nd nd nd nd nd nd CP-18 VAS Well CP-M N 7/27/1998 0.3 0 0 8.7 nd nd nd 0.3 CP-19 VAS Well CP-M N 7/27/1998 0.3 0 0 105.3 nd nd nd 0.3 CP-20 VAS Well CP-D N 7/28/1998 nd nd nd nd nd nd nd nd CP-21 VAS Well CP-S Y 7/29/1998 0 41.6 0 108.4 NP nd nd 41.6 CP-22 VAS Well CP-M N 7/30/1998 nd nd nd nd nd nd nd nd CP-23 VAS Well CP-M N 8/3/1998 nd nd nd nd nd nd nd nd CP-24 VAS Well CP-D N 8/4/1998 0 0 0 105.3 nd nd nd nd CP-25 VAS Well CP-M N 8/4/1998 0.7 0.7 0.7 21.1 1.1 0.7 0.7 nd CP-26 VAS Well CP-S Y 8/5/1998 296.1 164.7 1,233.9 126 nd nd nd nd CP-27 VAS Well CP-D N 8/6/1998 nd nd nd nd nd nd nd nd CP-28 VAS Well CP-S Y 8/17/1998 0 0 0 104.8 nd nd nd nd CP-29 VAS Well CP-M N 8/17/1998 nd nd nd nd nd nd nd nd CP-30 VAS Well CP-M N 8/18/1998 nd nd nd nd nd nd nd nd CP-31 VAS Well CP-M N 8/19/1998 nd nd nd nd nd nd nd nd CP-32 VAS Well CP-M Y 8/31/1998 0 0 0 1 nd nd nd nd CP-33 VAS Well CP-S N 8/31/1998 nd nd nd nd nd nd nd nd CP-34 VAS Well CP-S Y 8/31/1998 5.8 23 88.5 112.6 NP nd nd 23 CP-35 VAS Well CP-D N 9/1/1998 nd nd nd nd nd nd nd nd 

VAS

no. Project1 Site type Regional aquifer system Susc. cat. Date Percentage of young fraction 

CFC-11 CFC-12 CFC-113 SF6 CFC-11/ CFC-12 CFC-113/ CFC-12 CFC-113/ CFC-11 Average

Table 6. Percentage of young fraction in water samples from wells and springs in Virginia, 1998-2000—Continued[VAS, Virginia Aquifer Susceptibility study; Susc. Cat., Susceptibility category where "Y" indicates water sample exceeded chlorofluorocarbon (CFC) and tritium threshold values of 5 picograms pe

(TU), respectively and "N" indicates water sample did not exceed threshold values. CFC-11, (trichlorofluoromethane, CFC13); CFC-12, (dichlorodifluoromethane, CF2Cl2); CFC-113, (trichlorotrif

hexafluoride; NP, not possible; nd, not determined. See figure 1 for location of wells and springs]
Tables 55

CP-36 VAS Well CP-M N 9/1/1998 nd nd nd nd 13.9 nd nd nd CP-37 VAS Well CP-M N 9/2/1998 nd nd nd nd nd nd nd nd CP-38 VAS Well CP-D N 9/2/1998 nd nd nd nd nd nd nd nd CP-39 VAS Well CP-M N 9/2/1998 nd nd nd nd nd nd nd nd CP-40 VAS Well CP-D N 9/3/1998 nd nd nd nd 12.4 nd nd nd CP-41 VAS Well CP-M N 9/10/1998 nd nd nd nd nd nd nd nd CP-42 VAS Well CP-D N 9/10/1998 nd nd nd nd nd nd nd nd CP-43 VAS Well CP-S N 10/1/1998 nd nd nd nd nd nd nd nd CP-44 VAS Well CP-S N 10/5/1998 nd nd nd nd nd nd nd nd CP-45 VAS Well CP-S Y 10/5/1998 0 51.4 0 104.7 NP nd nd nd CP-46 VAS Well CP-S Y 10/6/1998 57.4 68.7 72.6 46.5 NP 68.5 49.9 68.5 CP-47 VAS Well CP-M N 10/6/1998 nd nd nd nd 23 nd nd nd CP-48 VAS Well CP-S Y 10/7/1998 nd nd nd nd nd nd nd nd CP-49 VAS Well CP-S N 10/27/1998 1.2 0 0 102.7 nd nd nd 1.2 CP-50 VAS Spring CP-S Y 10/28/1998 96.9 104.7 117.7 89 nd nd nd 93 CP-51 VAS Well CP-D N 11/4/1998 nd nd nd nd nd nd nd nd PD-01 VAS Well PD Y 6/28/1999 343.5 97.3 106.9 3,844.1 nd nd nd 97.3 PD-02 VAS Well PD Y 6/29/1999 1.1 37.6 3.7 107.6 NP nd nd 37.6 PD-03 VAS Well PD Y 6/30/1999 19.4 185.1 76.1 330 NP nd nd 76.1 PD-04 VAS Well PD Y 6/30/1999 15.6 1,203.8 61 4,938.1 nd nd nd 61 PD-05 VAS Well PD Y 7/1/1999 1,186.8 314.8 24.3 835.7 nd nd nd 24.3 PD-06 VAS Well PD Y 8/23/1999 27.4 266.5 518.5 71.5 nd nd nd 71.5 PD-07 VAS Well PD Y 8/23/1999 8.9 113.3 995.1 104.4 NP nd nd 8.9 PD-08 VAS Well PD Y 8/30/1999 78.6 88.8 359 124,138.9 NP nd nd 83.7 PD-09 VAS Well PD Y 8/31/1999 nd nd nd nd NP nd nd nd PD-10 VAS Spring PD Y 9/1/1999 170 133.5 220.7 81.4 nd nd nd 81.4 PD-11 VAS Well PD Y 9/1/1999 357.8 413.3 293 nd nd nd nd nd PD-12 VAS Well PD Y 9/1/1999 0.8 58.4 106.9 765.9 NP nd nd 58.4 PD-13 VAS Spring PD Y 9/2/1999 112.2 148.8 434.4 155.6 NP nd nd nd 

VAS

no. Project1 Site type Regional aquifer system Susc. cat. Date Percentage of young fraction 

CFC-11 CFC-12 CFC-113 SF6 CFC-11/ CFC-12 CFC-113/ CFC-12 CFC-113/ CFC-11 Average

Table 6. Percentage of young fraction in water samples from wells and springs in Virginia, 1998-2000—Continued[VAS, Virginia Aquifer Susceptibility study; Susc. Cat., Susceptibility category where "Y" indicates water sample exceeded chlorofluorocarbon (CFC) and tritium threshold values of 5 picograms pe

(TU), respectively and "N" indicates water sample did not exceed threshold values. CFC-11, (trichlorofluoromethane, CFC13); CFC-12, (dichlorodifluoromethane, CF2Cl2); CFC-113, (trichlorotrif

hexafluoride; NP, not possible; nd, not determined. See figure 1 for location of wells and springs]
56 Aquifer Susceptibility in Virginia, 1998-2000 PD-14 VAS Well PD Y 9/2/1999 81.9 130.7 183.9 147 NP NP nd 81.9 

PD-15 VAS Well PD Y 9/2/1999 nd nd nd nd NP nd nd nd PD-16 VAS Well PD Y 9/13/1999 nd nd nd nd nd nd nd nd PD-17 VAS Well PD Y 9/13/1999 48.3 49.4 598.5 101.7 72.4 nd nd 72.4 PD-18 VAS Well PD Y 10/18/1999 4.8 65.6 320.1 108.6 NP nd nd 65.6 PD-19 VAS Well PD Y 10/19/1999 51.3 76.8 132.8 105 NP nd nd 76.8 PD-20 VAS Well PD Y 10/19/1999 4 740.1 161.4 101.2 nd nd nd 4 PD-21 VAS Well PD Y 10/25/1999 2.1 8.9 371.7 101.1 NP nd nd 5.5 PD-22 VAS Well PD Y 10/25/1999 71.3 66.4 71.7 97.9 nd 63.5 71.8 67.6 CFC-11PD-23 VAS Well PD Y 10/25/1999 nd nd nd nd nd nd nd nd PD-24 VAS Well PD Y 10/26/1999 nd nd nd nd nd nd nd nd PD-25 VAS Well PD Y 6/27/2000 144.4 310 72.4 111.5 nd nd nd 72.4 PD-26 VAS Well PD Y 6/27/2000 37.8 111.3 55.2 0 NP NP 30.4 30.4 PD-27 VAS Well PD Y 6/28/2000 18.7 109.3 112.4 1,492.3 NP NP nd nd PD-28 VAS Well PD Y 6/28/2000 224.6 121.9 110 1,757.5 nd nd nd nd PD-29 VAS Well PD Y 6/29/2000 0.9 302.9 0 107.3 nd nd nd 0.9 PD-30 VAS Well PD Y 6/29/2000 7 35.6 48.5 103 NP 30.7 nd 30.7 VB-01 VBCH Well CP-S N 5/13/1999 5 4.9 0 42 4.6 nd nd 4.6 VB-02 VBCH Well CP-S Y 5/14/1999 3.8 0.9 0 115 nd nd nd 2.3 VB-03 VBCH Well CP-S Y 5/10/1999 0.8 111.5 0 62.3 NP nd nd 0.8 VB-04 VBCH Well CP-S N 5/12/1999 17.9 14.2 0 93.9 12.9 nd nd 12.9 VB-05 VBCH Well CP-S N 8/14/2000 nd nd nd nd nd nd nd nd VB-06 VBCH Well CP-S N 8/11/2000 0.3 2.9 0 100.4 NP nd nd 1.6 VB-07 VBCH Well CP-S Y 8/10/2000 19 6.4 79.4 103.3 nd nd nd nd VB-08 VBCH Well CP-S Y 8/9/2000 0 11.3 106.6 103.3 NP nd nd 11.3 VB-09 VBCH Well CP-S Y 8/16/2000 4.9 9.3 83.3 104.8 47.9 nd nd 47.9 VB-10 VBCH Well CP-S N 8/17/2000 0.4 2.8 0 106.7 87.1 nd nd 87.1 VB-11 VBCH Well CP-S Y 8/16/2000 7.8 4.8 34.1 100.1 nd nd nd nd VB-12 VBCH Well CP-S Y 8/15/2000 0.4 11.5 145 103.9 NP nd nd 11.5 

VAS

no. Project1 Site type Regional aquifer system Susc. cat. Date Percentage of young fraction 

CFC-11 CFC-12 CFC-113 SF6 CFC-11/ CFC-12 CFC-113/ CFC-12 CFC-113/ CFC-11 Average

Table 6. Percentage of young fraction in water samples from wells and springs in Virginia, 1998-2000—Continued[VAS, Virginia Aquifer Susceptibility study; Susc. Cat., Susceptibility category where "Y" indicates water sample exceeded chlorofluorocarbon (CFC) and tritium threshold values of 5 picograms pe

(TU), respectively and "N" indicates water sample did not exceed threshold values. CFC-11, (trichlorofluoromethane, CFC13); CFC-12, (dichlorodifluoromethane, CF2Cl2); CFC-113, (trichlorotrif

hexafluoride; NP, not possible; nd, not determined. See figure 1 for location of wells and springs]
Tables 57

VB-13 VBCH Well CP-S N 8/8/2000 nd nd nd nd nd nd nd nd VB-14 VBCH Well CP-S N 8/7/2000 nd nd nd nd nd nd nd nd VR-01 VAS Well VR-C Y 7/6/1999 17.4 0 106.4 2,543 nd nd nd 17.4 VR-02 VAS Well VR-C Y 7/6/1999 38.1 37.2 25.3 204.8 69.6 47.6 51 56.1 CFC-11/CFC-1VR-03 VAS Well VR-C Y 7/7/1999 82.5 100.9 434 479.7 NP nd nd 82.5 VR-04 VAS Spring VR-C Y 7/7/1999 104.9 103 2,511.7 721.5 NP nd nd nd VR-05 VAS Well VR-C Y 7/8/1999 81.1 113.5 467.1 240.1 nd nd nd 81.1 VR-06 VAS Well VR-C Y 7/8/1999 53.3 104.9 61,127.2 0 NP nd nd 53.3 VR-07 VAS Well VR-C Y 7/8/1999 35.1 82.9 355,348.5 444.2 NP nd nd 82.9 VR-08 VAS Well VR-C Y 7/9/1999 nd nd nd nd NP nd nd nd VR-09 VAS Well VR-C Y 7/9/1999 34.5 59 59.7 372.9 NP 59.8 25.4 59.8 VR-10 VAS Well VR-C Y 7/9/1999 80.7 122 83.9 nd NP NP 84 84 VR-11 VAS Well VR-S Y 7/9/1999 81.5 81.9 3,956.3 2,683.3 81.4 nd nd 81.4 VR-12 VAS Well VR-C Y 7/21/1999 85.8 279.1 93 1,031.5 nd nd 86.5 86.5 VR-13 VAS Well VR-C Y 7/21/1999 193.5 113 245.7 107.3 nd nd nd nd VR-14 VAS Spring VR-C Y 7/22/1999 23.8 32.3 1,918.9 1,062.1 NP nd nd 28.1 VR-15 VAS Well VR-S Y 8/10/1999 47.7 46.8 121,244.4 172.1 46.4 nd nd 46.4 VR-16 VAS Well VR-S Y 8/11/1999 nd nd nd nd NP nd nd nd VR-17 VAS Well VR-C Y 8/11/1999 95.8 124.7 870.6 775.8 NP nd nd 95.8 VR-18 VAS Well VR-S Y 8/12/1999 nd nd nd nd NP nd nd nd VR-19 VAS Well BR Y 8/24/1999 86.5 348.4 313.1 104.2 nd nd nd 86.5 VR-20 VAS Well VR-C Y 8/24/1999 74.2 79.6 203.8 94.7 NP nd nd 76.9 VR-21 VAS Well VR-C Y 8/25/1999 32.1 63.5 331.5 87,708.2 NP nd nd 63.5 VR-22 VAS Well VR-S Y 8/26/1999 38.5 48 218.2 100.1 NP nd nd 43.3 VR-23 VAS Well VR-S Y 8/26/1999 nd nd nd nd nd nd nd nd VR-24 VAS Well VR-S Y 10/27/1999 5.3 9 203.4 106.4 40.3 nd nd 40.3 VR-25 VAS Well VR-C Y 10/27/1999 2.8 15.6 68.3 109.8 NP nd nd 15.6 VR-26 VAS Spring VR-S Y 10/28/1999 79.1 91.9 115.3 104 NP 79.2 64 71.6 CFC-1VR-27 VAS Well VR-S Y 10/28/1999 51 65.5 54.7 108.2 NP 72.6 50.3 61.4 CFC-1

VAS

no. Project1 Site type Regional aquifer system Susc. cat. Date Percentage of young fraction 

CFC-11 CFC-12 CFC-113 SF6 CFC-11/ CFC-12 CFC-113/ CFC-12 CFC-113/ CFC-11 Average

Table 6. Percentage of young fraction in water samples from wells and springs in Virginia, 1998-2000—Continued[VAS, Virginia Aquifer Susceptibility study; Susc. Cat., Susceptibility category where "Y" indicates water sample exceeded chlorofluorocarbon (CFC) and tritium threshold values of 5 picograms pe

(TU), respectively and "N" indicates water sample did not exceed threshold values. CFC-11, (trichlorofluoromethane, CFC13); CFC-12, (dichlorodifluoromethane, CF2Cl2); CFC-113, (trichlorotrif

hexafluoride; NP, not possible; nd, not determined. See figure 1 for location of wells and springs]
58 Aquifer Susceptibility in Virginia, 1998-2000 VR-28 VAS Well VR-S Y 7/12/2000 1.2 6.4 2 14.8 NP 17.1 nd 15.9 CF

VR-29 VAS Well VR-S Y 7/13/2000 5.8 11.7 0 104.6 72.6 nd nd 72.6 CVR-30 VAS Well VR-S Y 7/13/2000 15.9 29.5 30.6 42.6 NP 30.7 nd 36.6 CFVR-31 VAS Well VR-S Y 7/17/2000 10.2 11.4 0 102.9 19.5 nd nd 19.5 CVR-32 VAS Well VR-S Y 7/17/2000 6.1 44.1 19 80.4 NP nd nd 6.1 VR-33 VAS Well VR-S Y 7/18/2000 74.9 112.9 85.5 nd nd nd nd 80.2 CVR-34 VAS Well VR-S Y 7/18/2000 12.6 51.6 53.3 145.2 NP 53.2 nd 53.2 CVR-35 VAS Well VR-S Y 7/19/2000 2.1 9 0 102.5 70.8 nd nd 70.8 CVR-36 VAS Well VR-S Y 7/20/2000 77.6 89.3 74.2 74.1 NP NP 78 76 CFVR-37 VAS Well VR-S Y 7/17/2000 31.6 41.1 41.3 181.1 NP 41.4 26.8 41.4 CVR-38 VAS Well VR-C Y 7/18/2000 51.8 76.8 70.2 0 NP 84.4 46 84.4 CVR-39 VAS Well VR-S Y 7/18/2000 27.4 30.3 34.2 51.2 65.6 28.5 24.9 42.5 CFC-11/CFC-12,CFCVR-40 VAS Well VR-S Y 7/19/2000 171.3 18 47.4 40.8 nd nd nd 44.1 VR-41 VAS Well VR-C Y 7/19/2000 16.5 19.1 109.4 100.7 36.8 nd nd 36.8 CVR-42 VAS Well VR-S Y 7/20/2000 80 495.2 83.7 469.7 nd nd 80.2 80.2 CVR-43 VAS Well VR-S Y 7/20/2000 10.1 29.6 29.7 124 NP 29.7 nd 29.7 CVR-44 VAS Well VR-C Y 7/20/2000 119.7 871.6 101.8 541.8 nd nd NP nd VR-45 VAS Well VR-C Y 7/25/2000 60.1 64.1 66.8 1,061.4 83.2 62.2 58.2 67.9 CFC-11/CFC-12,CVR-46 VAS Well VR-S Y 7/25/2000 30.7 37.2 39.1 129.1 NP 36.7 26.9 31.8 CFC-113/CVR-47 VAS Well VR-S Y 7/26/2000 96.1 108.8 611.2 762.3 NP nd nd 96.1 VTDW-01 VPI Well BR Y 9/16/1999 63.6 93.6 185.4 87.9 NP nd nd 90.7 VTDW-03A VPI Well BR Y 7/15/2000 32.2 50.2 95.8 101.7 NP 36.7 nd 36.7 CVTDW-03B VPI Well BR Y 7/15/2000 32.5 50.4 68.7 104.7 NP 44 nd 44 CVTDW-07A VPI Well BR Y 7/14/2000 10.1 69.6 70.6 190.1 NP 70.5 nd 70.5 CVTDW-07B VPI Well BR Y 7/14/2000 nd nd nd nd NP nd nd nd VTDW-08 VPI Well BR Y 9/16/1999 121.8 127.3 255.6 88.8 nd nd nd 88.8 

VAS

no. Project1 Site type Regional aquifer system Susc. cat. Date Percentage of young fraction 

CFC-11 CFC-12 CFC-113 SF6 CFC-11/ CFC-12 CFC-113/ CFC-12 CFC-113/ CFC-11 Average

1 Project designation: VAS, Virginia Aquifer Susceptibility study; VBCH, Virginia Beach shallow ground water study (Johnson, 1999); VPI, Virginia Polytechnic and State University fractured-rock h2 VAS Aquifer System designation: AP, Appalachian Plateaus; BR, Blue Ridge; CP-D, Coastal Plain Deep system (depths greater than 400 feet); CP-M, Coastal Plain Middle system (depths between 2

CP-S, Coastal Plain Shallow system (depths less than 200 feet); PD, Piedmont; VR-C, Valley and Ridge Carbonate; VR-S, Valley and Ridge Siliciclastic

Table 6. Percentage of young fraction in water samples from wells and springs in Virginia, 1998-2000—Continued[VAS, Virginia Aquifer Susceptibility study; Susc. Cat., Susceptibility category where "Y" indicates water sample exceeded chlorofluorocarbon (CFC) and tritium threshold values of 5 picograms per k

(TU), respectively and "N" indicates water sample did not exceed threshold values. CFC-11, (trichlorofluoromethane, CFC13); CFC-12, (dichlorodifluoromethane, CF2Cl2); CFC-113, (trichlorotrifluo

hexafluoride; NP, not possible; nd, not determined. See figure 1 for location of wells and springs]